Why can not use an exchange address for p2p mining? Most p2p mines (like this one) have the following norm:

"Don't use an exchange address!"

I am mining alt cryptocurrencies but I do not have a real interest in maintaining a separate wallet for every kind of currency. Why they do not admit addresses from exchange sites?
Many exchanges don't have a fixed deposit address for each client. 

This means that when you want to make a deposit, it will give you an address and it will log that you are depositing to it. However, when they don't receive anything on the address, the address expires and might be reused for another client. 
So when you use the address for mining, it's possible that the coins you get from the pool and are send to that address will never end up in your wallet.