I've seen these, but have had no desire to get one.
As others have mentioned, being inflated, the water will slosh around a lot, and you will lose room in your pack and always have that sausage feel.
As for the benefits of spraying, I get a pretty decent pressured stream out of my regular camelbak bladder. In fact, I used it to rinse out some deep leg wounds just a couple weeks ago. And, if you really need a pressure spray, just take of the pack off and push on it or kneel on it, or have someone else push on your pack while it's still on your back. I've done that once or twice, and got about as much of a high pressure spray as I'll ever need.