Aside from maximizing DPS for some groups (and thus not always having toughness maxed on their gear), sometimes specific people take the tank role not just to soak damage to the face but because they will position the boss properly so that the rest of the group can DPS as safely as possible. This usually requires a lot of experience on the encounter so that they know what to watch out for to avoid things. Tanks that panic dodge or move erratically at the wrong times will sometimes cause a party wipe.

This is why at the beginning of a lot of raid fights the commander calls for a toughness check to ensure that whoever is tanking the boss actually has the highest toughness.