I was playing a couple weeks ago and a controller said to me (paraphrasing) "I don't know whose idea it was to make controllers able to tank but I love it."
I found out what he meant when I turned out to be completely useless on the team with my tanker....well....useless as far as the supposed job of a tanker. I still intercepted single mobs that got through the controller but that's what I already did with my scrapper....helped with agro management.
Now, when I play my tanker, I feel like I am just helping with agro management...especially when there is a controller on the team.
I don't think any single AT should be able to replace another but currently I think that controllers can replace tankers.