You've disabled the carry sure, but you won't kill them. Meanwhile they can kill your allies because you're busy chasing the carry and not protecting them!
"if you lose when you've disabled the carry, then I'm not sure how you'd win with the carry actually in the fight
"
You win because your carry is better than their carry. By not chasing after the enemy carry you can protect your own carry instead.
I'm not saying never ult the carry, it's just not a rule tanks should stick to. Tanks are NOT anti-carries. They're tanks, eg. if it's ult the enemy carry alone vs ult the two melee going for your carry, you should always go for the two melee.
@owl: "What? No. I think he means that you ult the carry and your team (with you) then rapes the carry, after which your team kills everyone else on the enemy's team."
yes that sounds good on paper. But you make it sound as if the carry is standing completely out of position. Suddenly ulting the carry in a gank might be good but in a head on team fight, a good carry is in a good position and that is far behind the tank and the melee if the big ticket skills (malph's ult) haven't been used yet. Now if you have a few anti-carries/assassins on your team then ulting the carry to insta-gib might be ok. But otherwise, your team has to go through their tanks/melee first which then leaves your own carry exposed and increases the damage your team took.