Even with a mixer, the bitcoin address that the payment was sent to is still fixed. If it is for a porn site, now everyone knows what BTC addresses have made payments to a porn site. They may not know what the porn site did with the proceeds, but in this context, how important is that?
In this case, and most cases involving services, it doesn't matter what happens to the proceeds. What matters is the user's privacy. Say Bob signs up for that porn site, but doesn't want his girlfriend Alice to know. If the site uses the same address for everyone, then Alice could look at Bob's transactions and see that he made a payment to a porn site. However, if the site gives one deposit address per user, then Alice will see that Bob is making same sized payments consistently to someone at a consistent time interval. If they generate a new address then Alice will only see that Bob is sending same sized payments to someone, possibly different people because of the different addresses, and is sending them at a consistent time interval. With cases 2 and 3, Alice will know that Bob is paying for something. With case 2, she knows it is the same thing, a subscription to something. With case 3, she is unsure if it is the same thing, if the price is a common price, and she cannot be sure that the payments go to the same person.
Most sites use case 3. They generate new addresses for each deposit. They also usually don't sweep all of the funds into the another large address, but instead will send withdrawals from an address that another user deposited in. This helps maintain anonymity.
