An open skill based mmo results in more rather than less diversity in my view. In class based games balancing is rarely if ever perfect, and players adopt the few classes which work best en masse.
Even were time not factored in - no1 in the 2 years since Ryzom launch has mastered all skills let alone there being a herd of clone Masters of Everything - freedom to choose and pursue what is of interest to us results in greater diversity. Some skills such as elemental magic are pursued by most, but this in no way is clonish. Some like myself train it to for self-defense when digging - as an assist to my primary interest of crafting, while others train it to pvp, or pve or boss hunt. And while I have high elem skill, I heal in teams as I find healing more interesting than elem. Because we have an open skill based system here, a mage is not a mage same as every other mage in other mmos cuz we all had to be mages if we wanted to play the game satisfactorily.
Another mmo design myth is that if players can do everything themselves no1 will team. The opposite is in fact true as multiskilling facilates teaming. In other mmos can stand for hours looking for a healer looking for a healer yawn. In Ryzom anyone can heal so if short of a healer someone who usually melees or elem can heal and the team isn't unable to play as can happen in class based games. Ability to pursue any and all skills facitilates teaming across all levels too as long term players still have low level skills they can use to team with newer players.
Once u see an open skill based system in action, I'm sure u will join the majority in a mmorpg.com survey who voted for skill based rather than class based mmos. If anything, class based systems are the cause of the problems they are intended to address.