It seems to me that MMO designers have been creating class systems poorly starting with EverQuest. I consider EverQuest (and all the other clones since) a closed class system wherein you are pigeon holed into an archetype before you really know what you're going to enjoy - kind of like choosing a major in college. The only thing all the accounting majors out there are happy about now is the fact that they don't have to sell their man juice to put food on the table.
In my opinion designers have just become lazy and uninspired and are falling back on class systems that have been around since Dungeons & Dragons. But they aren't even doing that very well - at least in D&D you could be a fighter/mage/thief and enjoy all the benefits of those classes, you just leveled a lot slower. The only argument I have in favor of a closed class system is that the class lines are very well defined and designers may try to achieve "balance" between classes. There is probably some number cruncher at Sony who had an epiphany whilst giving their daily rim job that forcing players to start from scratch when they want to try a new class would make a larger profit. I didn't mind this as much when I was in college and had more free time than Casey Anthony - but now that I'm older with responsibilities and a child I don't intend to smother with a pillow I'd prefer to just stop playing than to start from scratch. After multiple expansions in an MMO this could easily mean 80+ hours spent mindlessly killing moss snakes and kobolds.
On the other hand, Ultima Online (the great grandfather of MMO's) has an open class system in which you can decide to change your entire skill set around on a whim. Tired of getting smashed in the face by monsters? Become a tamer and let your greater dragon take the punishment. Or just become a crafter and be a carpenter, alchemist, tinker, blacksmith, bowyer, tailor, miner, etc, etc.
The open class system never stopped me from creating multiple characters either though. You really wouldn't want to re-level blacksmithing just because your sword broke. People would have two or three mule characters and sometimes even multiple accounts so they can build templates that they don't have to tear down when they get nerfed. On top of that, UO introduced the concept of a soul shard which allowed you to transfer that skill to another character or just store it. That's good shit.™ I'm not sure why all the clones out there decided the closed class system was preferable over the open class system but I've had about enough of it. My Cleric in rift is almost 50, and I think I might enjoy playing a Warrior but I'm just not going to spend time creating a new character to find out.
A friend I started playing Rift with has one of each four classes, but none of his characters are even level 25. MMO's are handicapping themselves with closed class systems in that players aren't experiencing the higher level, and arguably more entertaining, aspects of the game because they are too busy trying out all the classes. In the end these players don't become part of a long term revenue stream because they move on to a different game that promises not to be a clone of a game from 10 years ago.
There are a lot of MMO's I have not played so I cannot comment on all the other class systems. I'd be interested to hear what you think as an ideal class system.