if you're slipping properly, and by that I mean using the opportunity to close the distance and punch, then I don't see how the guy jabbing at you could have room to throw a left hook.. especially if his fist has already gone past your head.
What if you buy their feint? Slipping properly is ok in theory but the majority of this forum isn't made up of world class pugilists so I think its quite a realistic suggestion that risk would come from slipping onto the hook.
I think the most important thing is to accompany the slip with a counter punch. When a fighter jabs their ribs are exposed for that split second where you should take advantage. Yes, it is much harder than it sounds, but isn't everything?