Two factors to consider:
- If a mage casts Dissolve, Dispel, Sleep etc. over and over again, he/she is not casting anything that brings him/her closer to kill an enemy -> noone is really closer to win after such kind of spell. I do not say those are useless spells, but: even if it is annoying and cripple your tactic towards killing your oponent, at least he/she is not killing you right now.
- Whenever a mage is switching bound spell, he/she is not doing something else.
So - if your opponent has Mage's Wand with annoying spell, you should focus on killing him/her - ether you will deal him/her some nice damage, or he/she will not use Wand :]