I guess... but Draco could have assumed that Daphne would have done something to Hermione, and taken away some of the edge that they would have had to save him.

but the again, hermione can now rush in at anytime to help while Daphne is distracted. but draco still shouldn't have put hermione in any dangerous position then what he needed to. daphne really could have hurt hermione more then the cage spell which just hindered her