Right. If the target somehow had flame +12, you couldn't ever cripple with DT, because every result would be Burn. You'd also be doing something like 16 damage: there's gotta be something left of your enemy to cripple it!
As it is, I can't (quickly) think of any way a creature could have more than flame +3, (plant + smouldering curses). DT is bad against plants anyway, because the piercing is wasted. Gotta pick the right tool for the job.