you're saying that you find it acceptable to blow 11 spells to kill a group of mobs but not 15? hmm.. personally, i find anything more then 3 or 4 a total waste of spellpoints.
to put it simply, casters are about AOE DOT spells when mana preservation is the focus, which it should be. if you're running around chain lightninging or fireballing everything then youre an idiot. firewall.. period. the new system helps firewallers more then it hinders them. immune to firewall? cloudkill.. 1 cast! extended ck drops everything. kite it for a while. the only thing left is golems and none of the other instant aoe's work any better on them anyway.
for single mobs instead of a group you should be enerydrain/fingering them. 2 spells. i would laugh endlessly at anybody who used more then 4 spells to take out a group of mobs.