you can use a mage weapon to melee fight also, and do so at a skill level equal to your adjusted magery score. Using this method, you can gain magery while fighting, the same as you would use a sword to gain swordsmanship. With mage weapons lacking Spell Channelling, I would guess this to be their only real purpose.

Aside from that, as you mentioned, you can cast spells from a lower circle to gain magery while using a -skill mage weapon that has spell channel.

Hope this helps!