[F. Gottwald]

most of the time, wizards should indeed stay in the background... but when your muscle can´t get things done, a wizard can usually walk through the opposition and mow down anything in his path:
few things should be standing up after a barrage of skull traps from sequencers, followed up with a few horrid wiltings if you´re high enough. project image allows you hurl spell after spell at your hapless opposition without losing them from your spellbook and is probably the single most powerful spell at your disposal.
in fights versus enemy wizards, don´t bother with debuffing - noones magic protections are flawless so try to find the way to their heart (be it weapons with elemental damage, non-magical weapons, magical weapons or, most often, area spells) or just take whatever they can throw at you while polymorphed into a mustard jelly, then laught at them madly when they try to hit you with a brittle stick.
don´t be ashamed of cowering in the background in everyday encounters that would be a waste of your supreme skills... when needed, you can show those petty little fighter types what real power is.

clerics don´t really have the opportunity of such power trips, but they are nice and steady performers: adequate in melee, often the primary tanks in my groups (since i prefer caster/fighters like jaheira and haer´dalis over straightforward butt-kicking machines) with an impressive arsenal of spells. gate doesn´t have its equal among wizard spells, nor has fire storm (assuming the xp cap of 2950000). clerics are your tactical warheads as opposed to theatre-level weapons of mass destruction, and they fit that bill very well - they won´t be able to take out hordes of enemies within seconds but are excellent artillery support (holy smite and hold person are very useful) and won´t need to rely on pansy things like stoneskins to survive a fight with a 4-year-old.