Actually, it just proves that you want to turn the game back into XI where people would just stand around in town til a healer leveled into their range and were looking to group. I'm not saying that everything should be done solo, but the option does need to be there. Like it or not, people enjoy casual MMO'ing. In fact, it's growing in popularity and the days of hitting a wall in progression and discouraging casual players is ending as the play-style grows.
This comes back to balancing issues. Should a solo player gain more skill/exp per hour than a group? Nope. Do they right now? Not entirely, but some classes make ranking easy when solo. If the player wants to rush their way up the progression latter he/she should feel the most viable option would be to group up. For a player who doesn't mind the rate of their progression should be able to solo and have the ability of survivability. See what happens? Both parties are happy and it's not a very extreme idea that holds a lot of unbalancing issues and we're not irrationally taking all forms of survivability away from players.
I think it's kind of condescending to the casual player base by saying they are not good enough to play an MMO if most cases than not they have to solo for a small amount of time. This is the problem with what you want out of an MMO, it shuts the majority of players out. We would like the game to succeed, Right?
...right?
Restricting the class does not make them unique. Unless, unique to you means to be totally useless or co-dependent to other classes. You don't have to take away survivability from one class to make another unique. It's unique in it's ability to perform it's task. If all forms of survivability were equal, then yes, that would then make the class that specializes in healing less unique.
Again, balancing issues.