I must admit to capo-ing up to keys like Gb when there are melodeon players around and I don't want them joining in on certain songs - mind you I've once had one playing G natural (sort of, his instrument was rather flat) to my Gb without seeming to worry. There are many times that you don't want someone else joining in - If you have a quiet voice (I don't) one enthusiastic Castagnari could easily drown you out. You might also be wanting a bit more control over the phrasing/tempo than you'd be allowed otherwise. It's easy to state at the beginning of a song that you'd like to do it solo.

On the other hand, if a musician deliberately tunes out of standard pitch, then they are also making it impossible for themselves to play in with the others - which might be a good thing for the others !