You have to remember that the World Cup is essentially a neutral-venue tournament, so it's not exactly apples to apples. Imagine if France had hosted the most recent World Cup, but played all their knockout games in another country. That would be silly.
MLS supporters who have their club qualify in the 5, 6 or 7 seeds may very likely not get a home game, even if they win the MLS cup. Their home support is essentially done at the end of the regular season. Plus you miss the drama of a two-leg series. Awful.