FF certainly means MC first is always viable. MC rush decides most games on LM, in my experience.
Ninja in, drop the hive, spit it and wait for reinforcements. Same for when you're breaking a marine relocate to a hive. I've used it when we've been on two hives and you're mc rushing the third.
Downsides - 40 or 50 res a pop, depending how you look at it, and no guarantee your team will MC in to finish it. It ENABLES easier hive defence, but does not make it certain.
Upsides - Hive defence relatively easy to organise, as long as there are enough MC to allow fast retreats to the hive. MC rush negates PG rushes, negates siege rushes. One alien can hit the hive long before sieges are even halfway built. Hear something being dropped near the hive? Attack the hive and get instant reinforcements.
Is negating siege/pg rushes a good or bad thing? I'm happy with it, because siege rushes are pretty hopeless if there's a few gorges in the hive, and the threat of MC reinforcment forces most marine attacks to be swift and decisive, which imho makes for a more enjoyable game for both sides.
TBH the only way we're going to find out how much of a crutch MC is, is by changing FF for a "test period", perhaps 1 weekend. Thats two whole days, ideally including a reg night, and then analysing the stats from there.