I have done tele turns in my AT gear for my brother's camera when the terrain was really flat. The reason is that a telemark looks more dynamic on film than a parallel turn in those conditions.
Ok, Ok, I guess I should have said, "no non-aesthetic reason". Maybe it's my tele background, but I don't have that much problem skiing AT gear paralell without the heel locked down. I'd worry about damaging the bindings with prolonged use (but them, I can break anything)