BS gives you the ability to add an extra socket into your bracers and gloves
Alchemy allows the use of Flask of the North in arenas, also they receive a passive ability called Mixology which doubles the duration of flasks and elixirs in addition to having an increased effect of 25~50%
Leatherworking gives you a superior bracer enchant (i.e. +76 Spellpower, +102 Stamina & some resistance enchants)
Tailoring gives a cloak enchant which is a chance-to-proc effect of, in your desired case; +295 spellpower (other options are 400 AP proc/400mana restore proc)
too lazy to look up the rest of the stuff like engineering, but maybe you should just do what I did; do a wowhead search =p