The point of Figure Independent Injection is that it solves two problems at once: It allows more than just the first figure in a scene to be injected (unlike stock V3/M3/Freak) and it simultaneously defeats crosstalk, without null figures.

However, if a scene using more than two figures is saved as a pz3 (Poser Scene) file, crosstalk will exist upon reload unless there are nulls between the figures. So it might be a good idea to use nulls with FII, especially if your Poser is prone to crashing.

The primary drawback of an FII figure is that it is incapable of crosstalk unless saved as a pz3. This doesn't matter if the scene is going to be naked people. But, it means clothing morphs cannot chase a base figure's morphs, because that is an aspect of crosstalk. And since M3's love muscle is a conformer, like a clothing item, it will not follow M3's FBM's if M3 is converted to FII. This does not mean a clothing model's (or love muscle's) morphs will be disabled. They will still work. They just won't work automatically in Poser 4/PP. They'll need to be set to match, manually, which shouldn't bother P5 users because P5 doesn't crosstalk, anyway.