Yes.That is a bad thing. Not everyone wants to play all classes. There is no reason to punish them by creating situations where the class they love to play is left out of an instance. Giving a player a choice and forcing them is two different things.
You don't punish players for not playing all classes. You reward them for playing all classes.
You don't punish new players for not having all classes maxed with great gear. You reward those that played long enough to max all classes and have great gear on all of them.
And also, not everyone has the time to play the game every single day. By the time they can grind out each class, they are already going to miss out on a lot of content.