So, let's talk about classes first. Classes make sense when you have potentially millions of players, all who have different desires and preferences, playing classes that most likely have different specializations they can go into. It is probably the easiest way to balance things- not only between players and mobs, but between classes and possibly even factions for some MMOs. That's pretty much the norm for MMOs these days. But- why do so many old-fashioned tabletop RPGs still being produced today still have classes? Aren't classes an archaic construct? What are the advantages of classes when you don't have to wrangle millions of players (or even dozens) and the mathematics and game mechanics are extremely simple compared to what goes on under the hood of a computerized MMO?
Now, on to the second issue. Generally, I don't think there is really any such thing as an "MMORPG". There are MMOs, but they ain't RPGs. Role-playing means doing what is expected of your class, filling your class's role, and nothing more to the typical MMO player. One possible exception: LotRO. Most people are such huge fans of the lore and of Tolkien that a lot of people role-play, or at least stay in character. That MMO actually has 'lore Nazis' who will gripe at people who misuse public channels and report characters with questionable names.
You know, for that matter, I would not call console games like Fallout or GTA or Elder Scrolls true RPGs...to me, if you aren't playing with other human beings and you aren't staying at least somewhat in character, it's not true role-playing.