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.
Oh BTW, I still have and use my 77mm one, I'm selling my 72mm because I sold most of my primes and the ones I have left, I rarely use...
Again, Primes = inconvenience, anyone who shoots studio with a 50L or 85L is a bit silly... you need versatility, speed and convenience 90% of the time to make your life, and your clients lives easier..
(Assuming we're talking about 35mm camera systems)
2) That depends! If you shoot jpg and you are not familiar with / not willing to do post-processing then you'll need a tool to define the grey point. Canon's AutoWB is known for getting confused under mixed light conditions, here you NEED it.
If you shoot raw then you DON'T need it.
3) If you use it then it has value for you. Whether you need to use it.. see 2)
b) Depends. I got one to try because a long time back i was psyched by a camp superior and i thought i will only shoot jpeg and I wanted to get the color balance right. Then after that really short lived purchase i realized I still love shooting and working with RAW.
c) The worth of an accessory for a hobby is really up to the buyer. I got it at a higher price.
You actually can still use it for raw as a reference point...
In fact, you should probably do that, because you're mood changes dramatically when the colorbalance is off... ESP if you're using colored gels on your lights.
This of course is not necessary if you have time to fool around with raw files, but it's faster that way..
The cheapo disc is good and bad, good because it gets the job done for cheap, which is good...
Bad because with the ones i've seen, it doesn't have a diffusion material to diffuse the light evenly, it's just a plastic piece that muddles the colors, and you don't get as good of an accurate average reading..
However, some of the cheapo discs go for a steal, so you're set if you can't make up your mind if its worth it or not, just get a cheap one and see how you like it. It'll be relatively the same.
2) No. I have a cheaper imitation of sorts. Works well, but I've almost never used it. Not under work situations anyway.
3) No. For reasons as above. Hardly used, and it's just a piece of plastic. Some people have found reasonable success with a Pringles can cap!