I certainly don't prefer CZ or Asha over genuine diamonds, but as far as diamonds go, I like them white and clean, which is very expensive. When it came to my wedding set, the color and size and clarity of the diamond were important to me, and DH and I agreed to spend the money on rings that we would both wear every day for the rest of our lives (until I get him to upgrade my rings!). But with earrings, I still like a clean, white look, and don't want to spend the money.
As far as earrings and pendants and fun rings go, I like to be able to wear pretty jewelry that may or may not be "classic." So mixing genuine diamonds with CZs and Asha just makes more sense. I don't have money to throw around any way I want, so I'd rather spend more on bags and shoes than on jewelry. I do spend extra money for 14 or 18k yellow or white gold, but don't see the point in spending the extra cash on diamonds. Also, if you look at "affordable" diamonds vs. really good diamonds, there's a huge difference. I'd pick a CZ over a "spit diamond" any day!
I fell in love with a cross pendant at Tiffany recently. That thing was over $7,000. Beautiful, yes. Classic, yes. But $7,000? If I knew it would be something I'd wear every day no matter what else I had on (like my wedding rings), I might be able to justify the price. But I know myself too well. I like to change my necklaces, and don't usually wear any one more than once or twice/week. So maybe I'd be better off buying a CZ pendant for now, and spending $100 or $200 for CZs set in gold, or even platinum. One day, maybe I'll be able to afford to spend that much money on one piece of jewelry that I'll wear sometimes. Right now, though, I just can't justify it.
The other thing is that young couples just starting out really shouldn't put themselves in debt for an engagement ring. There's a lot of societal pressure to keep up with the Joneses, and I think a simulated diamond is a much wiser choice than breaking the bank for a ring. A simulated stone can always be replaced with a genuine diamond later.
Then there is the conflict diamonds argument. I pretty much stay out of that one, but there are people who are VERY committed to doing everything they can to keep from supporting the diamond trade.