For me, I think it helps convey expression. No, not jumping around, or moving esscesively, but i've met some good players who move around quite a bit. 'Specially Jazz players. :)
I think it can get ridiculous, but i think you should do whatever makes you MAKE MUSIC the best. I think its similar to playing something very sad, and really thinking sad, really trying to express it. Its the human touch, expression that computers can't mimic, not even the very best of the best. And i think thats becuase its not something writen down, or can be explained. If they want to put their soul into the song, so be it, as long as it doesn't detract from the music, and even if it does, who says that music is meant for the audience. I dont play music to preform it, i play it for me. There's lots of music that's written for the sake of music, not the audience. If i get some joy out of it, i can do it however i want, I'm the one who's playing the music, not the people/person watching.
sorry, thats really long... i'm way too longwinded :D