I think its true that skills can be practised in a dream, only you probably cant get physically fitter since you cant move at all.

But for skills which can be simulated using mostly thought and require some degree of accuracy (skiing, golf, archery), these can be enhanced my practising in an LD.

Twice I have used dreams for bettering my art, so I guess it will be the same for music. Practise makes perfect, and you have 8 hours every night which you wouldnt otherwise be using