Other RPG classes and cliché character types

In a single player system, classes are a good way to start your player off in the right direction. Not only would the Rogue have a different motivation for starting the journey than the Paladin, he'll have a different set of skills. Even if you go with an open ended leveling system, having a balanced set of abilities to begin with helps guides newer players in to a playing style. "Oh, so those are the things I want to level up to be a good axe guy" or "so this helps healers more than fire magi?" As a compromise, if you really hate classes, offer presets that someone could glance at and apply in their own way.

As for a multiplayer game... Well, go play TF2 and then play Brink. In TF2, you can notice a class and know their general purpose instantly. Even though there are a selection of weapons (Demoman can go Sticky or Demoknight, Medics with their different mediguns, Minisentries or L3s on Engineers) you're never completely in the dark. For Brink, you never really know what you're being hit with until you're dead, and without even consistent silhouettes you can't tell how you and your team mate can work together. You have a bit more time to react in RPG style games than in FPS, but the principle applies. "LFG Someone who can heal" is much more vague than "LFG Cleric Group Healer".