In our group, casters usually concentrate on 2 things: buffing the party, and/or damage spells.
Save or Die spells almost never work. It is just too dang easy to make saves. When things start getting dangerous, our casters throw out damage, because even a save results in half damage, or it is an orb spell and a roll of 2 hits the touch AC of most big, slow, powerful monsters. (Dragons are a real sucker for Orb of Force). A rod of substitution is great, you can turn all your spells into cold for example, making that Fireball useful against the Red Dragon. Got a save or die spell vs. the BBEG - pray the DM rolls a 1, otherwise you are not doing much.
In addition, damage spells are assisted by the melee guys in the group, as everyone is working towards the same goal, to kill the monster through sheer damage - meaning even a made REF save does something to the thing.