I would never, as either a DM or a player, try to "enforce" any sort of balance among the roles. Ever. I'll make them aware that they're potentially impeding themselves, to make sure they're okay with it, but that's it.
4E is not so fragile that it's going to fall apart if the group isn't perfectly composed. Yes, they'll probably have some troubles that a "proper" group wouldn't, but that's their decision. As the DM, I'll tweak some of the encounters to account for the group's composition, since that's obviously what people wanted to play. I won't cater to it completely, but I'll keep it in mind.
And if it still turns out to be an issue? It's a self-correcting problem. If the party is too weak to function, eventually one or more characters will die and be replaced by someone more effective. But until/unless that happens, so be it.