I thought of one possible reason that would make deontology "justifiable" in consequentialist terms. Those classic "my decision has negligible effect by itself, but if everyone made the same decision, it would be good/bad" situations, like "should I bother voting" or "is okay if I shoplift". If everyone were consequentialists, each might individually decide that the effect of their action is negligible, and thus end up not voting or deciding that shoplifting was okay, with disastrous effects for society. In contrast, if more people were deontologists, they'd do the right thing even if the effect of their individual decision probably didn't change anything.