It's an effective way to achieve market segmentation without having to ask your customers how rich they are, and you get the benefit of finding out additional information like their address, email etc. The principle is similar to coupons on cereal boxes, anybody can get the rebate/discount if they go to the effort, but people who are cash rich/time poor are less likely to do so than those that really need the money.
Joe Spolsky wrote about this and various other pricing mechanisms a while back, I like to reference the article every few weeks. It's well worth a read.
Now, if you're retired and living off of social security, $7 an hour sounds pretty good, so you do it, but if you're a stock analyst at Merrill Lynch getting paid $12,000,000 a year to say nice things about piece-of-junk Internet companies, working for $7 an hour is a joke, and you're not going to clip coupons. Heck, in one hour you could issue "buy" recommendations on ten piece-of-junk Internet companies! So coupons are a way for consumer products companies to charge two different prices and effectively segment their market into two. Mail-in rebates are pretty much the same as coupons, with some other twists like the fact that they reveal your address, so you can be direct marketed to in the future.