Cast times are there for a reason - to balance skills. If there were no cast times, why would anyone use the weaker skills? You could just spam fireball or flame choir or whatever.
There are better ways to buff the Mage class, by giving them new, diverse skills. (And of course fixing magic resist but enough has been said about that)
For example, Mages only get 3 DoTs (correct me if I am wrong) -
Sustain Doom: It's a pretty good skill, rather extensive use of MP, but that's not too big a deal - a priest can dispel it though so it rarely lasts for it's full cycle.
Meteor Storm:This simply sucks in every way. I mean, a 2 second cast time, average damage, and the DoT is simply pathetic.
Flame Shock:Yeah, the DoT is great, it's instant cast, has a slightly lower range than other skills but meh that's no big deal, but it's UM only. Obviously, UMs should get more skills, but this is the ONLY useful DoT (imo).
This is just a small example, but there are far too many outdated skills. Since players can now achieve tens of thousands of HP, can link more and more stats with each new addition of content, and so many other classes have had their skills buffed (fighters/warriors have Rising Uppercut, I really hate this skill - it's basically just a sheer damage boost - completely spammable, with a debuff; makes no sense to me how this isn't considered OP).
I could go on, but you get the picture.