I use a lappy and sometimes wish I built a rackmount system. The reason I went with the lappy was the screen, keyboard, mouse, etc... makes for a much bigger footprint than a lappie. Sure they make smaller keyborads and trackballs, but I have recorded in some tight spots where I could have never gotten all that stuff into the spot. I've worked in spots where I had to fold up the lappy and set in on it's edge next to my other gear. I had to hold in one hand while making any adjustments. Luckily you don't have to touch your computer when multitracking except to start and stop recording. Venue owners aren't typically fond of you taking up that much realestate. One Idea I did have that I almost jumped on was a mac mini and a smaller touch screen. I think thiw would work reasonably well and be small enough to transport and use in a tight spot. There is als a big differance in throwing a lappie bag over your shoulder vs lugging around a big rack, keyboard, and monitor.
All in all, I would do the lappy again. Just my .02
Matt