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.
Yeah that ^
Keeping 100% of your keys is extreme. I guess there are some people who keep all their money in cash under the pillow. But guessing the majority keep most in a bank/invested some in their wallets and some change in the car. So keep majority in cold storage some on your phone/hot wallet and maybe few bucks for tips etc...
I'm not firing up my wallet and copying pasting address to tip a buck