Bossing the Flanks
So let's start with a quick run down on the interplay between wide midfielder and full-back in modern soccer. The wide areas of the pitch are far less dangerous than the centre, so wing play comes down to exploiting space in those wide areas to provide some mechanism to deliver the ball into the penalty area. Since the wings are less important to a defensive team, there's a lot more space to play with, hence seeing creative, fast players rather than the big battlers we so often see crashing the centre (our closest analogue to the latter archetype is Nate Jaqua). Beat a man wide and yard upon yard open up to explode into; beat a man in the box and some other defender is right on top of you and liable to kick the ball off your feet.
On defence, this area is covered by the full-backs, who are typically faster defenders in order to close down both the ball and any possible passing lanes. Wide midfielders occasionally track back to provide support, but it's rare to see even world class wingers who are capable of doing more fouling the opposition - they're there for their abilities on the attack rather than their defensive game. This leads to attacking full-backs being very useful in overloading a flank. Get a local 2 vs. 1 situation and you're suddenly giving the opposition considerable problems, assuming one's capable of doing something with possession wide. Teams will often cede the wide areas in front of the box to the other team's attack if they're confident that they can shut down attacks that emerge from such locations (an excellent example of this is the recent Premier League match between Chelsea and Arsenal), which isn't as hard as one might think: there are essentially only two attacks that are possible with the ball outside. Play one is to drive down the touchline and cross, and play two is to cut inside with the ball towards more dangerous areas in the hopes of playing short passes or shooting.
The first option is more suited to the full-back, the second the winger. Why?
Strengths and Weaknesses
The answer to both lies in the skill and pace of the wide midfielders. When given the choice between pressing a winger or preventing passes a fullback, there's no real option for a defender: they must close down the more dangerous attacker, because consistently allowing the winger a free run into the box would be a very good way to get demoted to half-time orange slicer. This means that an overlapping run by a fullback when a wide midfielder is on the ball in an advanced position will be relatively unopposed, and plenty of space towards the corner will be open. Involving the fullback in the attack almost inevitably leads the ball towards crossing position, and a team looking to exploit the ability to overload a flank should see this happen routinely. The second option, the push into central positions, is of necessity the winger's because they are, on the whole, much more capable shots and passers than their defensive compatriots. In essence, the dynamics of the modern game means that full-backs cross and wingers push the ball into the danger areas. This is, of course, a generalisation: wingers have to have the option of crossing if their backs cannot support them fast enough (this happens when the winger is very, very, fast - think Aaron Lennon), and you'll sometimes see full-backs take potshots from the wing, although they're usually quite bad at them.
The Antithesis of The Standard: Seattle's Left Wing
This norm then raises an important question of the Sounders attack. Why are crosses from Leo Gonzalez so rare and those from Steve Zakuani so frequent? Does anybody remember Leo crossing the ball last season? I assume he must have done, but such occurrences were so few and far between that I literally have no idea how good he is at delivering the ball into the box. Similarly, Zakuani pushed outside so often that by the end of the year his marker was cheating him, playing half turned to their right in anticipation of where he'd push the ball (this decision invariably proved to be correct).
When your left midfielder is providing all of the service down that flank when you have a left back of the calibre of Leo Gonzalez, your team has a problem. When said left midfielder's crosses are as dicey as those provided by Steve Zakuani, it's a double issue. Don't get me wrong: I think Zakuani's one of our best players, but crossing the ball is not his strong suit. Something strange is going on down that wing, and it likely contributed to the Sounders punching well below their weight offensively. Why don't we see the usual full-back/winger combination play down our left side? I have two hypotheses.
1) Leo is simply not fast enough to keep up with Zakuani in order to provide the requisite support.
2) Zakuani's tendency to move the ball outside when confronted with a defender cuts off the space Leo would be crossing from, forcing him to play deeper.
I don't think it actually matters which one of these is true. They're both resolved by Steve Zakuani adding a new weapon to his arsenal this offseason.
Drive For Goal
Zakuani is a converted striker, and although it's clear that he loves space, he may have forgotten the advice of a legend (not that he's alone in that company):
"When forwards attack from wide to inside, they are far more dangerous. It's funny when I see centre-forwards starting off in the middle against their markers and then going away from goal. Strikers going inside are far more dangerous, I think. When [Thierry] Henry played as a striker, and sometimes when Wayne [Rooney] does, they try to escape and create space by drifting from the centre to wide positions, when that actually makes them less dangerous."
-Sir Alex Ferguson, Manchester United manager
If option one above is correct, Zakuani is, for all intents and purposes, a striker who happens to start with the ball fairly wide. On fast breaks, where support from one's team's deeper-lying players is more of a luxury than an expectation, the player on the ball must act like a forward. Standard rules of attack (cross, cross, cross, repeat) rather go out of the window when a team can find themselves two on two and bearing down on goal. It is in this very situation that Zakuani should shine, and yet he's ineffective here precisely because he lets himself get pushed wider and wider until he's forced to cross (and on fast breaks there're not many targets to hit on a cross anyway). Cutting inside the defender presents a much bigger problem for the opposing side, especially as Zakuani's speed should result in most defenders getting left behind. So, option one means that Zakuani should probably driving for goal more often than he does.
Option two is that he's simply not giving Leo the space to make an overlapping run, and that's even more clear-cut than what's just been discussed above. You can't expect players to function effectively when two of them are put in the same place on the field, and there's no reason to expect good results when Zakuani is moving into space where Gonzalez should have ownership. Cutting inside with the ball and laying it off to an on-rushing full-back gives more space and time for a cross to be delivered.
Either way, I think it's clear that Zakuani's signature stepover and push left trick isn't going to cut it. Having the ability to bring the ball inside will open up whole new avenues for the team's attack on the left side, which seem to have been sorely neglected over the past season. Having Zakuani scaring the opposing defence by running towards the goal instead of away from it while Leo Gonzalez provides the crosses? Yes please. The best part is that as a 21 year old, Steve Zakuani still has plenty of learning to do, and once he does, that side of our attack will go from ho-hum to a fairly devastating offensive weapon.