I think they use those for aerodynamics also. But yeah they turn up in spring and summer. Not much outside those seasons.

I find them very annoying, they tend to be very inconsiderate. Cycling on the road where bike lanes are available, and when they are on the bike lane they tend to be agressive to normal cyclists if they don't move along as fast as they want.

They're not all like that but many of them are, which leaves a bad taste as soon as you spot these groups of wannabe lance armstrongs :P