I don't disagree, I just think that most service providers find the 'first come first served' model easier to implement. Having said that I'm struck by how this seems to matter less and less on Twitter, eg many celebrities have somewhat random-seeming handles.
And if you're referencing me from your phone, you could alias me as @knowsnothing and change my icon to a koala if you wanted to. That offends me less than me having to come up with @awesome_sauce314 to use some photo sharing service, which I'd need to share among friends, coworkers, family, strangers, etc.