Do you have any statistics on this?
I know plenty of people who download and never buy. My roommates are two examples of this. In fact we got into an argument about this very subject last night.
Most people I know who are 'music fans' -- that is, constantly going to shows, finding new bands, posting on forums, etc -- buy most of the music THEY LIKE after downloading. There are exceptions, but the majority do this. Obviously this can't really be applied to every music fan everywhere.
A lot of times when I read posts like this, people seem to forget what the intended effect of sampling is. You decide if you like it or if you don't. Believe it or not, there are lots of times where people don't like a band or album! Why would you buy it? I'm not saying that's what you think, but it's what a lot of people seem to imply with similar posts.
PS some people still buy records based on album covers! Found out about Cultes des Ghoules recently because of the cover for Spectres Over Transylvania. Best black metal release of 2011 and I'd have never picked it up if it didn't have the great cover!