I must admit it, I want a smart phone. This probably stems from the fact that I'm tech type of person, but I think I've come up with two practical excuses to actually get one.
1) I use Google Calendar, Gmail and Google Talk - all of which have applets which can be downloaded onto my phone. Though Gmail and GTalk would mostly be for fun, the calendar would be really useful. I used to have a palm, but that system really only works with a single computer. Now that I'm jumping between my work and home machines, syncing all the time is too much of a hassle. Google, which easily shares between machines, chose to be mobile through smart phones. Since my calendar really is a large part of my life, I think this is a good excuse.
2) Sometimes I'm traveling and don't have access to WI-FI. In those situations I could tether my cell phone to my computer. With a data plan, I would get internet access through either an EDGE or 3G connection. Then, my computer could pick up the signal my phone puts out. Thus, my phone acts as a conduit for the internet. I've done some math, and if I canceled my current internet plan and solely went through my phone, I would actually save money. Think about it: if you're spending $40 for your phone now plus $40 for internet access, paying $60 for a phone and data plan actually saves you money. True, the connection isn't as fast, but just think how cool that would be. This is definitely a good excuse.
So those are my two reasons. I think they're pretty solid. Now I just have to get over the $400 hump to just get the device...