Choosing a key to accomodate a voice makes perfect sense to us singers. Choosing one to accomodate another instrument also has merits--I understand that it's generally easier to play a fiddle in D or A compared to other keys, for example. It makes sense to me that if one has, say, worked out a guitar finger- or flat-picking arrangement in one key, playing in a different key (sans capo) would be like starting over.

Some people maintain that certain songs "sound best" in certain keys. I recall a vigorous discussion here on Mudcat sometime in the last year or so about this. I remain unconvinced. I remember seeing three guitarists, with no other instruments and no voices involved, all put their capos on before playing a certain tune. When I asked why they all capoed up 2 frets instead of just playing open, they said the song sounded better in A than in G. Maybe to their ears, but I can't say I've ever noticed such a thing.

Nevertheless (and back to Harpgirl's query), I've been in many a jam where certain songs are expected to be done in certain keys, that's what everyone is familiar with. Unfortunately, I don't recall that key being F for any songs other than what I mentioned earlier.