I'm sure the coaching staff has it's reasons, they need a DE in the worst way and I'm positive they've considered anyone who could remotely help out.
As someone who doesn't know the mechanics of DE/DT play, I'm just curious as to why a smaller guy like Whitlock seems to be a better fit inside than at end. Is it because his advantage is speed and leverage, so he's better off going against bigger, slower interior linemen?
I really think it has more to do with the fact that he is familiar with DT and doesn't get defensive snaps to learn DE. I would not count him out of being able to do it, athletically though.
He looks good as a limited role player, no need to go out of your way to find something he CAN'T do.
The Giants have been looking for a DT to get QB pressure up the middle forever, and he has. Even in his very limited role he then becomes an impact player.