It would not at all be useful for Bitcoin mining. However, it may be useful for other digital cryptocurrencies, such as Litecoin or PPCoin.
Not practical at all. Most probably the users would be just pissed of. Right now it is not even practical to mine with GPU-s, not to mention CPU-s and for sure JS.
Regarding the question: "if I put the user in as part of a mining pool, ..."
No, it is not important that he would find a hash. It is working this way. Your all workers are calculating hashes (does not matter if you find the right one or no). If a miner mined something at that time, the miner calculates how much work you have done and gives you the amount of bitcoins. So if you contributed 5% - you will get 5% of 25BTC and in case of some pulls fees. If pull mined nothing, you get nothing.
So in total, if you do not want to piss of users, do not do this.