ease of switching between apps is one reason. If I have MSN running in the background on my BlackBerry, and I'm in another app, and I get a message (through a customizable ringtone, not an annoying popup), I just invoke the app switcher (similar to command-tab), pull up MSN, respond, and use the app switcher to get back where I originally was. The whole thing takes maybe 15 seconds. That's how long it used to take my 3G to just open BeeJive and have it all loaded and ready to use. It's like the difference between being in your computer's browser and command-tabbing over to Adium to respond, then command-tabbing back to your browse, versus being in your browser, having to quit it, launch Adium, respond, quit Adium, and then re-launch your browser.