You can say that real multitasking is useless on a smartphone, you can say it's useless on a 3.5 inches screen, you can say battery lasts 5 mins, anyway....

N900 isn't a smartphone in the usual way of terms, it's more like a tablet, like a mid, like a mini-pc!

I love multitasking ala N900, there are situations where you need it (you're on the phone/chat and need a search on google or maps or need to send via email a photo just taken, etc...); and for sure you'll impress your geek friends by showing applications running simultaneously on task manager (like videos playing or games/emulators running flawlessly)