Every account has a place in their profile for a BTC address, and many people have something in their signature that mentions they can send tips to a certain address if they liked their post.
As it was mentioned above, there is no real need to integrate a 3rd party app for any bitcoin payment, tipping included
There's likely 'no real need' for many things but it is a nice addition. How many people will actually boot up their wallet to copy and paste an address and send a tip here? A third party application just makes it more convenient and a bit of fun, not to mention you'll actually know where the money came from. I think it can also act as a nice 'plus 1' feature but one where people can actually reward with something of worth to show their appreciation.
There are a lot of people on the forum that strongly believe that you should always 100% control the private keys to your bitcoin. Any tipping feature that is "on forum" would obviously not allow that. Some people could leave a tab open for their blockchain.info wallet, or could leave multibit open all the time.