I like a mobile sleeve, but I also want it to be clean. I don't see how, if my sleeves were messy, they would be any more mobile. In any event, I am happy with the mobility I have. I wouldn't want any less, but I don't think I need any more.
If the original line in the draft below would produce a clean-fitting undersleeve, the red line is what I mean by cutting a long undersleeve. The shaded area is a bit like a gusset allowing much more movement; if I were cutting for a conductor I would adjust the sleeve like this. It's messy but you have more movement. I've seen it in a lot of the A&S stuff, but maybe he's cleaning the sleeves up these days. If Astaire was abler to dance around without his collar moving, it would have had more to do with this type of sleeve cutting than a 3" back neck.