I certainly take Will's point about different keys having a different sound and feel to them, but it's a bit harsh to blame it all on the box players. I take my fiddle along to a good few Old Timey sessions (not usually places that welcome melodeons), and they're even more restricted - you can be playing in D for a hour or more. That's partly becasue the fiddle style involves heavy use of open strings, but also because banjos are even less flexible than boxes, so you tend to stay in one key until - by mutual agreement - all the banjo players retune, and you then spend an hour in G instead.

If Peter Kennedy hadn't demanded G/D boxes, we'd probably have ended up with a nation of C/G or C/F melodeons, like they use on the continent - in which case the choice of keys would be just as limited, and the fiddle players would be complaining. I find it hard to believe, incidentally, that French musicians would complain about tunes in C, since that's home territory for them.

If you play harmonica it's easy to take along a boxful but, although more than a few melodeon players have boxes in C/F and other keys, you don't always want to clutter up a packed session room with them. Pretty well all G/D players can find their way around E minor, and plenty can play in A, Am and Bm as well. However, I agree with Steve that inclusivity is an important aspect of sessions and, while it can be refreshing if someone dives off into a non-standard key from time to time, doing too much of that is similar in terms of etiquette to leading off technically difficult tunes that most of the room can't play.