Consider this...
ALL caster specs have their own form of aoe support whether it be damage or utility. So, they are all viable in group fights.
Wizard - aoe nukes (megabolt + pulse), mobility (blink + port), control (root), sustainability (replenish), high single target damage (fireblast + bolt). - Best solo caster spec in my opinion.
Geomancer - HUGE aoe nukage (blaze + icy aura + pulse), control (deepfreeze + icebolt), sustainability (flame shield + fire armor + reflect) - AMAZING damage output in group scenarios. Probably the hardest to kill of the caster specs because of the sustainability and control.
Necromancer - HUGE aoe nukage (Plague on its own absolutely wrecks), mobility (hellgate), sustainability (battery + drainsoul + become undeath), control (web + despair), single target damage (Decay) - Overall a great class because of the kiting ability and utility at hand (my personal favoite). :3
Beguiler - ranged aoe ability (plaguebomb + pulse), mobility (teleport_to_other), awesome support/utility (summonfood + safefall other + wisdom + dispel + purge + might), control (piggify), only single target ability (fireball). - I think that this class is taken for granted honestly. If I wasn't bard, i'd probably play this class. It does a great job in group fights, yet it is extremely underplayed. I would consider this class as more of a support than a dps.