I tend to think it's psychological.
I was adamant that I wanted real diamond stud earrings to match my wedding set, but I had no idea what size diamonds to get, so my husband suggested getting a pair of CZs in 0.5 or 0.75 carats and see how I felt about the size before getting the real thing. Well, after screwing up my nose at the idea, we found a pair of CZs for $15 and quite frankly they have been great. If they scratch and go dingy then I'll just replace them with another pair of CZs or get the real thing. Right now I'm happy with CZs because I'm not so scared about losing them.
At least I now know I want nothing bigger than 0.5 carats, so we've learnt a lot from the $15 CZ experience.