I just got the Canon 10-22 for my 40D, and as mentioned above, it gives a very different perspective to a shot - something you couldn't replicate with stitched pic's at 17mm.
As to whether you 'need' it - check out what it will do for you and if you want that for your photography. At it's wider end such a lens is a special tool. I love it for landscapes (esp. with interesting skies) and for emphasizing objects - but get CLOSE!
I recommend having a look here too: http://www.kenrockwell.com/tech/how-...ide-lenses.htm