I'm selling one on the Buy Sell forum now, and I'd say you don't NEED it, it's not worth EVERY penny, and you surely could get great photos without it.
To be honest though, I use one when I'm shooting Jpg, I put it on take and photo in AV or TV mode, then go to manual WB, and click on that photo, then when I'm shooting indoors it's pretty much decided that that'll be the proper white balance, and I can safely fire away all the jpgs I need... If the lighting changes within the room, or I run outside or something, then I just pop the thing over my lens and do it again... It's quick and does the job. If your doing a series of raw files in the studio, just use a 18% grey card before you start, or pop on the expodisc and fire like your taking a shot then set it..
There are other uses for it, but thats mostly what I use it for...
You can always shoot raw and adjust later, you can always shoot jpg and trust your camera (although thats a bad idea)....
Anyway, it's not needed, but neither is autofocus for still life, but people still buy AF lenses
You get what I mean, it really is a convenience thing.
Best,
ADam