If it's a song then people can join in with voices and sing in the chorus. For a guitarist to try to identify the key by noodling and then plonking away with chords that don't fit if they don't know the song is very off-putting to the lead singer and anybody else who wants to join in with voice/harmony. Tunes are easier because you can usually identify the key straight away by where the melodeon playher has his/her fingers or the guitarists their capos and chord shapes.