I agree with JudeL; environments with polyphonic singing (like rounds, part singing, etc) make it very difficult to remain in your own part unless you are experienced. FIE helps a lot. As for being unsanitary, even people who do the real FIE rather than the cup-behind-the-ear are not necessarily doing anything unsanitary; your ear has a little flap in front of the earhole and when you put your finger "in" your ear you are really pushing on that flap so it closes your earhole. Your finger does not really go "in" your ear, but someone looking from even a slight distance would not be able to tell this.

I rarely if ever use FIE these days, but when I used to attend round singing groups I would absolutely have to use it or I could not stay together with my part. It's true that it's fairly ridiculous to use it when singing unaccompanied and alone unless there is a lot of noise or the acoustics are really bad. But I wouldn't presume to accuse anyone of using this just for show. Certainly if you know Martin Carthy you know he wouldn't do something like this just to imitate Ewan MacColl; it must be helping him or he'd lump it. That said, I'm sure SOME poseurs do use it for the wrong reasons.

As for singing with one finger up your ass, I guess it might help you hit the high notes ;-)