I think it really depends on the people you play with and the difficulty of the challenges in the game. I don’t think any class should be terrible but getting passed over because another class does a few more percent of DPS than yours is really a problem with the player base more than anything else.

Examples:
Anarchy Online. 12 classes. 2 of them “real” tanks, two more “optional tanks”, which were used for tanking when the “real” ones were not available. The medic als “full” healer, but several other alternatives available. And several classes dedicated for damage. Luckily for the game, the tank classes were exchangeable as were the healers up to some degree.

Warhammer Online. Each side has 3 tanks, 3 melee DDs, 3 ranged DDs, 3 healers. Picking the most obvious negative example here, physical ranged DDs were the only kind of DD not able to pass/penetrate armour. While having absolutely great style, the affected classes were rarely played, so perhaps the effort to implement them at all would’ve been invested better in another place?

My point of view, still sticking to this example, is that no matter if you want to deliver your ranged damage by throwing spells at the enemy, shooting arrows or hurling grenades and firing your blunderbuss, each ranged class should be viable. Playing styles (quick fire vs. slower abilities with higher punch) may vary, but each class needs to be useful, else it’d be better not to implement it at all.

A PvP focused endgame is similar to a raiding one in that it requires balanced classes for the end result to be more diverse. The coolest class ever IMO may be the Sushi Chef, dual wielding a butchers clever and a live salmon, but if I get destroyed by every class in the game (due to class weakness, not my poor PvP skills), I won’t be able to stick with it for long.

Then there’s the question of 1v1 balance vs group balance. Perhaps my Sushi Chef is aweful at 1v1 but is the best DPS available with group help, is that balanced? I’d say yes. Others may say no, but at least the class has a role, some roll. Throw all balance out the window and if the Sushi Chef sucks at 1v1 and is worst group damage, they’ll be played by very, very few people.

All of the above is assuming a competitive system of progression with dps meters, gear treadmills, rated pvp or other systems of measure. Any game with any focus on rewarding endgame performance with items or skills to enhance endgame performance will suffer without balance.

Now, the opposite to what I’ve detailed above is a game that doesn’t reward anything related to the actual performance of the endgame activity – meaning no gear treadmill, no pvp rating, etc. I’m not saying it couldn’t have gear, it just wouldn’t be on a performance treadmill and instead would be crafted or random world drop or economy based. How do you make a game, which lacks a measurable competitive activity, fun? I know people did enjoy open PvP in WoW before any “pvp system” was implemented, but for how long would that keep their attention? How long would they continue playing compared to those that played for years while raiding and doing arena?

This is all considered apart from simply enjoying the leveling game. Perhaps a game could focus on the leveling game (and not worry about balance across classes as long as each class can level somewhat efficiently) by allowing something like the SWTOR legacy system from the onset. It could have something like an exponential experience boost based on the amount of alts somebody has within a certain level range. Something like 1.5x exp for leveling two chars within 5 levels of each other, 2.0x exp for leveling three chars within 5 levels of each other, etc. That would really push focus to the leveling and alt game, but then what happens at end game? What happens without balance? Do people just quit after leveling all those alts?

I honestly don’t see an alternative to games with limited but balanced classes. Then again, DAoC had a ton of classes and had plenty of players bitching about their weak or useless class, but most people seem to hold the game (myself included) in high esteem – even if it is through rose colored glasses, so maybe I’m just way off base and the fun in class diversity is enough to trump the need for the most efficient method of competing (raiding or pvp).

Don’t roll a Cleric and expect to be high DPS, don’t roll a rogue and expect to heal, don’t roll a caster and expect to go toe to toe in melee range.

In EQ early days if you planned to play solo a lot, there were limited classes to look at, new games nowadays all classes can solo to level cap. I feel this is a step backwards and reduces the social glue of MMO’s which in turn lessens the life of games. No MMO in the past few years has held long term appeal for me beyond a couple months.