With MLS having very closed finances, and Forbes only doing a report on the 2007 revenue streams, judging financial impact is rather difficult. The sole set of numbers that can be looked at are attendance. Those that think the DP has an impact might be surprised, as the Beckham effect was big, but everything else is tiny.
Certainly there are media impression issues, certainly there is a level of merchandising; but those numbers are closed.
The impact for attendance though has varied through the years.
In 2007 the impact of the DP was a 13% increase in attendance for the clubs that added one during that first season. Yes, the clubs that added DPs were generally better attended than the rest of the league (7% over league average for '06, and 11% over in 2007). Yes, the rich teams got richer for 2007.
For 2008 a few more teams tried the magic beans, and the impact went down. For their own attendance they stayed stable with a 0% movement. They did build on league average by 2% from the years, but it was still down. KC had their tiny stadium and a DP that year, and the Beckham effect was minimized.
Recession hit 2009, and muddies the waters even more than the British model does. Teams that went the DP route in 2009 lost 11% of attendance from 2008, they lose 3% in relation to league average (Seattle was adjusted out, as they had only a single season). Maybe the DP teams held their own a bit more versus the league during the recession (2008 DP teams at 13% over MLS v 2009 same teams at 10% over MLS).
When we have already seen that the on-pitch impact of a DP is minimal, and now we find out that the attendance increase is marginal; does the DP as a system really make sense? In the NBA the "Bird Exception" really helps a club as it increases the talent on the floor dramatically. In MLB one great player can make a huge impact (see the Twins, Blue Jays, Padres) as so much of the battle is 1-on-1. In the NFL, unless it is a QB the impact of a single extraordinary player is nearly nothing (see Terrell Owens).
In soccer a single great player just doesn't get the touches, and they don't have a similar player to hit with a dynamic pass. Having a single player, or two, or even three isn't going to make an impact. Think of the great goals from build-up that you have seen. Do they not involve at least 4 players? That's what MLS needs, at least four world class players on a team at a time. That's how they go from the 15-25 range in league strength to the 8-15. It wouldn't get them to Big Four territory, nor Brazil, nor Argentina, but it would come close to Mexico, Japan, and the middle of that 2nd Tier in Europe (France, Russia, Belgium, Netherlands, Portugal). The only way to do that at this time is to let the 4-8 money making teams to spend that money rather than pocket it, but to only dent parity rather than destroy it would be through a luxury tax style system.