As I understand it, there is a difference between abilities like x10 click and +1.8K% click. The multiply abilities (like from harem girl affection tiers) stack, and if you have a lot of them can skyrocket you into some powerful advantages. The +% abilities on contract girls seem to be independent of those multipliers, so while +1000% sounds like a lot, it might just be that you're outpaced by the multipliers, so it's less useful for you.
I remember someone else on the forums saying the contract girls are useful for newer players who are catching up (who don't have as many of the harem girl abilities unlocked yet). That's a good thing to have in the game! Just depends on where you're at.