I'm pretty sure there aren't a lot of people commuting on a TT bike, but when I do commute by bike, I do ride in excess of 20 mph on the flat. A good fitness hybrid is more than capable of such speed, a good cruiser probably isn't. I have used a cruiser as a commuter when I lived in New Orleans, it's a great bike for riding on truly awful roads. When I commuted on a hybrid, my commute included train rides on two different systems (Caltrain and BART). A cruiser would have been a terrible choice for that as taking up and down stairs, etc. would have been a nightmare. I didn't even consider it.
Point is that the hybrid was actually the ideal bike for that commute and it was also fun to ride when I wasn't commuting. The folding bike could have been easier to carry on the transit, but I didn't even know such a thing existed at the time (1995 or so), and I don't think I'd have enjoyed riding it around the Bay Area as much as I did my nice hybrid. I know people do such things, but there aren't a lot of them.