Actually, SmartPet does defend, just not automatically. Part of "The Button" features have it react according to what is happeneing when combat is initialized. To that extent, if you target your party member, it will assist and attack upon keypress.
But I digress, a new version of PetDefend is surely valuable if the automatic features of it haven't had it guts ripped out by Blizzard's nerfing attempts. Otherwise, I have planned to create an AcePet mod similar to SmartPet one day. Yes, not right around the corner, but one day.