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. I really don't think copying and pasting an address is that big of a deal to someone when they care enough to be giving away money.
Most people won't care that much about keeping a tiny amount of btc with a third party to tip, but if they do then those people just wouldn't likely be interested in using it anyway (also sometimes it's just not possible to control your keys eg on an exchange). I'd certainly tip people if it was implemented but I'm not going to log into my wallet every time to do so just to send a bit of dust. The transaction fees wouldn't be worth it and that's where microtipping comes in.