The "getting out the laptop" overhead is a killer for me. I will often go an entire weekend at home (when I have work needing to be done, etc) without switching from iPhone to laptop because I just couldn't be bothered getting out the laptop and firing it up. To be fair though, part of that's probably because my laptop is 5-6 years old, standby doesn't work and the battery life is average.
There are a lot of times when I need to SEE my email without composing email. I read a lot more email than I reply to, or I will need to see something but won't need to respond immediately, or I will log into email to get tracking info for packages, etc. I wouldn't use it for email all the time, of course. I'm not going to write a novel on the thing. That is rather impractical. But for accessing email when I'm away from the desk, it would be better than, say, an iphone, and less cumbersome than a laptop.