The thing with the Argyll vest and short sleeve shirts is that it's more comfortable to play in. Full dress (spats, full plaids, doublets etc) is totally uncomfortable to play in and very hot as well. You've got to remember that playing the pipes is quite a workout. The tartans that a lot of bands play are quite common tartans as well but it cost a lot of money to outfit a band and to use rare select tartan is an extra cost. As far as most bands are concerned it is the music that you are playing that is on show not the uniform that you are wearing. Some solo competitors are pretty elaborate like Gordon Walker and the Late John D. Burgess but I think most pipers would agree that if you want to stick out like them you'd better be able to play like them too!