I think it depends on what you mean by optimization mechanics here. I don't think one has to have an optimized party to be survivable, but you may have a different take on what optimization is.
If you have a party of 5 warlocks, then yes, there is going to be a survival problem with this group (especially as one warlock's curse in now way helps any other warlock).
But so long as the bases are covered (and it is not *that* hard to do this) then there should be plenty of room within those broad limits (a leader, a striker, a controller, a defender) to find reasons to stay in the group that also fit the player's inner vision for a character. If character concept A doesn't work (this time around) try character concept B, or C, or D.
I could only see it as restrictive if enough players in that group each has one, and only one, character concept, and they were not compatible with a survivable group.