I'd generally think it's foolish to buy GOTY/SE if you already own a copy. I'd feel terribly ripped off if that version contains more content, sure. But I won't go buy a new instance for it, regardless. In the modern days of DLCs and whatnot, I'm sure that can be circumvented now.
Never if I own a copy - or if I do, I sell my vanilla copy. I did that with Fallout 3, as the GOTY version was so much cheaper than buying the DLC separately. I'll do the same when Borderlands comes out, as I refuse to pay the asking price for the DLC. I didn't buy the LBP GOTY edition as the included content was kind of dumb (outside of the MGS level pack). If I didn't own the game before it would have been a must buy, but the sheer fact the game's DLC consists of costumes and stickers makes it an easy pass for existing owners.
Morrowind's GotY version was huge. I didn't get the game until years afterwards, but I don't think it would have been much of a question with all of the content that was included in the Tribunal and Bloodmoon expansions.
I only really buy Special Edtions of a game when theres worthwhile exclusive dlc eg. the Masterchief armour in Fable 2, But I'd only buy a GOTY edition game for something like Fallout 3 or Oblivion when ALL of the dlc is on a disk to get the full game Experience
I usually don't buy GOTY editions of games but I'm thinking of doing it for LittleBigPlanet and Fallout 3. I already have both of those games in my collection but I want to get my hands on all of the DLC Fallout 3 has to offer. I'll probably end up selling my regular editions on craigslist or something like that.