I was under the impression that when a spell has a % amount greater than the cap, it is intended to help counteract getting debuffed. Say, for example, haste is capped at 100%. You have an ability that grants 110%. Normally you will only get the limit of 100% since that is the cap. Unless a mob debuffs you. If he slows your attacks by 20%, your not at 80%, but 90%. That's why a master puts you even more over the max cap limit. It is basically trying to make debuffs on you negligable. That was my impression anyway. I could very well be wrong and if so, then I agree, it's senseless to have spells that have no upgradeable purpose.
Ellestil