Builder Huts are always a good investment! No matter whether you are willing to spend money on the game or not: Get all builders before spending gems on anything else! Even if you doubt being able to keep all builders busy at higher TH-levels, it's still a good idea to have as many builders as you can afford. Here's why.
Whenever you get a new TH, there's plenty work to do. You'll easily keep your builders busy even if you do not play 24/7.Â
Upgrades take longer at higher levels (ie. 14 days for Tesla 7, X-Bow 4 or Inferno Tower 3).Â
An unemployed builder is not a disaster. Use him to max-out walls and thus invest your gold in your base rather than leaving it for attackers!
More builders means more flexibility. In times you play excessively you'll make faster progress and that's one of the key factors to success. And hey, it's fun! =)