That's certainly true about comptuter controled mages, who sit around all day waiting for the PC's party to show up so they can instantly fire off all their premeorized spell triggers and contingencies. But for in-party mages, who can't afford to rest after every single battle. So he rests, only to find that he picked the wrong spot to rest, so now a pack of orcs appears and kills the helpless mage.
I'm not saying I don't like mages: I played an all mages (most were dual or multiclassed) party in BG2. All were at the XP cap, when I decided to fight Drizzt's party just for the heck of it to see how my mages would fare against tough fighters. Needless to say, I learned pretty quick how slow casting spells once per round is vs fighters that are attacking at 4x the speed
. It's not just that battle though: they also struggled against the drow because of high resistance (I used Tensor's Transformation a lot). The same applies to Suldanessar, because of the magic resistant golems.
Point is, mages are useful, but they definately have their limitations. Fortunately, for those of us that like playing characters that don't depend a whole lot on equipment, there's always the godly monk class