You think?
I think that they will sell out for a couple of games this year (home opener for certain) but interest tends to fall during the season. I don't think that their average fan really understands the MLS playoffs/CONCACAF and the rest of it. Even the hard core amongst us here, there are probably a significant portion that never watched a single MLS playoff game this year.
Fans they are attracting are used to table formats where the games mean everything. Last year we had a debate around comments that certain games weren't "must win." With the playoffs going the CFL route now with 12/20 making it in (that's 6/10 MLS vs 6/9 CFL) , it seems like less of an accomplishment. (I know big thing to say for a fan of a team that hasn't made it ever… yet). Particularly if you go all season to get knocked out in a single game.
I think there will be an initial hype but that will fall off through the season. People may want to follow a player, check it out, but long term sustainable interest in a team in the MLS will be a hard nut to crack.