Maybe in RIFT this is true. Not so in other games.
DAOC: Light Eldritch Stun nuke nuke. Elementalist spam. Warlock bombs. Casters were great, and it probably screwed the archetype for the rest of MMO history based on how hard people cried about it. The best PvP class MUST be something that beats your face with a giant phallic symbol, anything else isn’t fair.
WoW: AP-PoM-Pyro in Classic. Frost from there on out, with periods of Arcane being shiny. Both could easily solo PvP. Whatever you may think about PvP class balance in WoW, it is by far the best out there (minus Guild Wars, but that doesn’t count)… which is probably more of a testament to how awful PvP balance is in general, but still.
EQ2: Summoners were bar none the worst classes in the game for PvP from Classic to Kunark (I gave up on Nagafen after that, so maybe Necros became gods, idk). In terms of Mages, Wizards, Warlocks, and Coercers were probably the best (in that order), and were pretty good in general, but still weren’t anything compared to how amazing the Rogues were.
WAR: bright wizards lol.
Conan: Mages sucked and were outshined in many cases by DPS Clerics. (Whatever the lightning one was).
RIFT: Sadly, they’re just bad for the majority of the game. Anything I can do you can do better. Jack of all trades, not even good at any. Pick your corrupted meme and run with it. You’re pigeonholed into Chloromancer (getting nerfed in 1.1) or Archon (a buff bitch) in PVE because your DPS is pretty pathetic. Spell coefficients are depressingly low, so your gear does little for your damage – a new weapon for a Warrior may give him 50% more damage, while a new weapon of equal quality for Mage is lucky to do 5-10% more.
And yes, I’m bitching about MMOs, mostly because I’m a damn fool and always pick a caster when it’s clear the current trend is to grossly favor physical damage, especially melee. The ONE TIME I tried to change my ways it totally backfired (I wanted to be a Spiritualist in Aion but, knowing my first choice is always the weakest, I went Champion. Turned out Champion was godawful and Spiritualist was amazing. lul).