It reads very well on the iPad so I don't think they need one. The iPhone on the other hand would certainly benefit, it's a pain trying to read and answer questions on one if them.
Smartphones are great but there are limits to how easy they are to use. Looking at some websites entails enlarging the image because the type is so small. You then lose the sense of where you are on the page. Having said that I wouldn't be without my BlackBerry.