Paul Kedrosky ran a domain ownership survey, and concludes:
It is a tri-modal distribution, however, with there being a subset who own no domains, another cluster with 2 to 5 domains, and yet another cluster of people who own 10 or more domains. For the sake of argument, I'll characterize the three groups as people who don't care about domains, pragmatists who have a couple for their own reasons, and pros & speculators who own many domains for investment purposes. Infectious Greed: And the Survey Says .... You Like Domains
Purely on anecdotal evidence, I agree with the three groups (I fit squarely into the the pragmatist group), but disagree with the cut-off points. Kedrosky put 10 or more as the number which made someone a pro/speculator. I have more than 10 domains, based solely on the fact that it's cheap ($9US for full-featured domain management, somewhere around $3US for just registering) and easy (use Blogger or a myriad of other easy to use services and get a site up very quickly).
And in part, it's about topical focus and brand. I was discussing with Troy how he wants to structure a new site, which is actually more like 5 or 6 sites. Each site becomes a filter/tag/container for topically related content. Much like a Technorati tag (or other aggregation URL) shows content slices from everywhere, people are using domains to indicate the same thing.
My current recommendation for buying domains, for which I get no bonuses whatsoever, is still Namecheap.