Strangely, my experience has been that if one person is playing a given class, no matter how differently you could play that class, anyone else who was thinking about it immediately says something to the effect of: "Oh, I can't play that class, then, I'll pick something different."
Like, somehow, having more than one Fighter, Cleric, Wizard, or what have you would be a problem. Some classes I could almost see, like, two Paladins might slightly redundant with the Aura, maybe, but in most it just seems to open up new avenues of play. Like if you had a party with three Rogues and a Druid, suddenly stealth missions and gaining surprise over enemies could now occur more frequently. Two Wizards opens up scary spell combos. A group of archer Rangers might start all combats at range and make mincemeat of melee enemies., Etc, etc..