"Weak demons"??

Temptresses shouldn't be the core of your army. If you spend too much on them early on, you might lose other key units, like Pit Lords, for example. Temptresses are expensive, and building them in the early stages of the second week can be very unprofitable. Besides, if, GOD FORBID, the mages attack first (and if they split into 4 units, this is a very real possibility), before the Temptresses, the result will be predictably disastrous.