Okay, a case for more complicated phones:
A lot of times I'm called on trips where I'm only gone for a night or two. Ever since the Pismo blew up, we're down to one computer at home. So we have the choice of me taking the 12"PB with me or giving my wife access to Internet/Email.
But with, say, a SonyEricsson P900 I can surf the web, answer emails, take pictures or video, listen to MP3s at the gym, play games and even watch pre-taped television shows. Throw in a bluetooth keyboad and I can even get some serious work done with writing scripts and the like. So it effectively replaces not only my PowerBook, but my digital camera and iPod as well. Sure it doesn't do any of those as well as individual devices, but it does fit in my pocket and lets me travel light.
Having said that, We recently bought a new phone for my wife and went with a stripped down Nokia because she doesn't want, need or use any of the extraneous stuff.