Or at least hyphens. WordPress.com, at this point, is running into a scaling problem: There are so many wordpress.com blogs in existence, that if one grabs a word out of the air, really taking almost any word at random, and tries it as a wordpress.com username, somebody probably has it already. I've tried this as an experiment, and it does, indeed, seem to be true. So, our frustrated would-be newcomer tries using a pair of words, and sometimes will find that those combinations have been grabbed, too, but might also run into another problem. When two or more words are run together, what results can be a little difficult to read, difficult to remember, and ugly.
Personally, I like underscores in urls, because one's eyes almost overlook them; having those is the next best thing to being able to stick blank spaces in an url, I think. But some people, I guess, don't like those for some reason. Having hyphens (dashes), at least, would make a lot of urls a lot more decipherable and reduce the "what is that" factor that is going to start being a problem as the number of wordpress.com blogs climbs up in the millions.
I suppose that new users could append numbers to their ids, but those aren't so easy to remember either (as anybody who has forgotten a locker combination knows), and to some people, they seem to be a blow to the soul, especially when they discover that their own names have already been taken. Who wants to be thought of as being a number? We all know that each of us is but one member of a very large community, often easily lost in the crowd, but the guy who gets to be johnsmith45619 is really going to be getting his nose rubbed in that, especially if there was a johnsmith45618 when he signed up. He might not enjoy that. Word combinations, though, strung together, will take a lot longer to all be used up; there are more words in common use than there are names in common use. The user jackjohnson23 is going to be just another guy named Jack Johnson, but the user tortillas-and-chiles, if our hypothetical Mr. Johnson chooses that name, sounds more like an individual. It's not logical, and it's not a huge issue, but little things like that have an impact, and they affect the experience people are having.
An alternative, if everybody deeply hates hyphens and underscores: additional domain names. Network Solutions leases them for $30 a year or so, so I'm sure wordpress can afford a second one.