I think this'll be a big plus for us more hardcore fans. Let's face it, if a team has a core of 4-6 players that they don't want to trade, and the rest of them have already been with 2-3 teams, it's hard to design blockbuster trades that don't involve players going back whence they came, or that involve the same player(s) over and over again. I, for one, will be very glad the league won't have a core of Roy Lassiter-types that get traded every other year, because he's one of the few players who both has value and has not grown roots yet. And we won't have to see Richie Williams bounce back and forth into and out of DC. Etc.
All it means is 2 more teams than during the TB/Miami years. And it's not like MLS trades were any less "incestuous" during those years... sure it will mean more options in terms of trades, but MLS will continue to funciton in this "incestuous" way until Single-Entity is gone for good and clubs can sign their own players to their own contracts (limited by a hard cap and all the roster regulations in place today) and sign and release and trade players as they wish, with no behind-the-scenes mechanaitons.