One of the reasons I use an iMac rather than a laptop is that I never know if the client will have a decent or any screen for me to connect to.
I did consider a Mac Mini with a 17" screen, but then realised it was easier to cart an iMac around as it had better specs than the available Minis at the time.
I was originally carting my Mac Pro with 30" Cinema Display all around the country in the back of my M5, but I'm sure the travelling didn't do the computer any favours. What finally settled it was having to work in Central London or out f the country where driving was not feasible. So I tried a few laptop options in the interim and realised I'd have to carry a keyboard, mouse and monitor to guarantee my comfort anyway, so a 17" iMac wasn't that great a difference.
I've since upgraded to a later i5 iMac, which was only available in 21.5+, so it's a fair bit heavier than the old plastic 17" one.
Also, not having a laptop means there's no temptation/pressure to work on the train/plane.
This is what I cart mine around in, and having it on your back is quite useful on the Tube for making other commuters give you space.