Without wishing to over-egg the original argument, here's another example other than "The Sweetness Of Mary" one which I've quoted.

There's a great tune in Andy Hornby's "Winders Of Wyresdale" compilation called "The Physiognomist" - it's a 3-parter in Bb-F-Bb and, as I said in another thread, great for the tenor guitar. And it sounds fine in that key.

Now - I like sessions very much, and I like it when everyone joins in as much as possible. To me, that's what it's all about. BUT (and it's a reluctant BUT) - if I play this tune at my next session, I'm probably going to be a soloist, which I don't really want.

An alternative, then, is to transpose it into D (say), so that everyone - melodeons and concertinas included - will be with me. But, d'you know, I like the tune in its original key and don't want to play it in a different key. And... if such transposition keeps getting done again and again, then - there you go... Everything sounds the same and you lose the integrity of the tune somewhat.

Or perhaps I'm just nit-picking. :-)