I am purely speaking from my own experience and I can't speak absolutely... but every city I've ever lived in has had a few random cabbies driving around that are far outnumbered by the cab companies - whether this is yellow cab or something random like Boston's Metro Cab[1]. If you ever need to call and schedule a cab to the airport you'll be calling into a dispatch center - rather than just arranging it with a dude. My impression is that street hires are a pretty small proportion of the taxi market.

1. http://metro-cab.com/