Each Clone Vat gives you about 1 more new pop every 100 months. For most playstyles, in the long run you'll be thankful for all those extra pops (unless you are going super-tall or playing an extremely long game, the game will already be over by the time you have 'too many' pops). Not employing anyone is a good thing (again, unless you've reached the point where you've really run out of things to do with your pops), because for the most part you'll be putting them on new colonies or half-empty Ecumenopolises, where there are plenty of jobs to do and not enough pops to do everything. The only building I'd give higher priority to on a new colony is a robot factory, as it gives about twice as much pop growth as a clone vat (albeit at a greater cost).
Incidentally, if you suddenly have an opportunity to conquer a bunch of planets, then clone armies are great: they're like regular armies except they cost a bit less and can be recruited three times as fast. With clone vats everywhere, there's no need for long sieges: during the war, you can just churn out armies all over the place and conquer planets WH40k-style, by sending more bodies than the enemy has dakka.
You can always replace the clone vat with something else later if you're approaching the carrying capacity of your planets.
