It really matters not whether the kilt is a traditional or a contemporary, but jackets normally worn with trousers just do not look 'right' with them. Basically they are too long and, especially when viewed from behind, tend to give you a somewhat 'deformed' appearance. It is all a matter of proportion. Chances are that you would not be wearing a sporran with a contemporary kilt - but you might, and so it would be best for the jacket to be 'cut away' to allow for it.

I never have, nor ever would, wear an 'ordinary' jacket with any kilt - a proper kilt jacket (formal) or a leather 'bomber' jacket (casual) usually keeps me out of trouble.