Usually when I'm visiting my clients, I lug along a computer bag filled with various tools along with my MacBook Air. The rest of the time, I'm either in my office with my iMac, or away from a Mac enjoying myself. Unfortunately, Mac issues tend to arise at the worst possible times, and so there are many situations where I only have my iPhone available for support.
One of my tricks of the trade is to talk with clients while looking at their Mac screens using a remote support tool, and the iPhone screen has always been too small to be useful. When you're trying to control someone's computer through Jaadu VNC or some other remote control software, screen real estate is very helpful. I can see using the iPad for remote support in many situations where an iPhone just won't work.