Steve Davis has a pretty good peice up taking about the MLS Designated Player rule, a three year experiment that is coming to an end officially, although all indications seem to indicate that it will continue. Davis notes that some people in and around MLS are worried that the addition of a second DP will create a small market/big market split like that which exists in baseball.
With a strict salary cap in place and also not likely to disappear even in a new collective bargaining agreement, a small market/big market split seems unlikely. However, there is a much greater concern that would need to be addressed if a second DP were to become the norm.
If clubs are willing to pay for a second DP, they must--indeed they are morally obligated--to ensure that all players, even the most junior development player is paid a living wage of no less than $50,000 per year and must be willing to guarantee those contracts.
From a purely academic sense, I have no objection to a second DP, but you cannot have two or three players making six or seven figures a year and then have players making less than $20,000 a year. But if owners are willing to part with seven figures a year on two players, they need to make sure they are paying their full squad a living salary for a professional athlete. For a league that has demonstrated, not unwisely, a concern for the bottom line, having to DP's and a group of four or five junior players making less a year than a DP makes in a month is a moral outrage.
So if owners like those reportedly in LA, New York and Seattle are willing to pay a second DP, those same owners had better be lobbying their peers vociferously for a minimum salary and guaranteed contracts.