i don't know what is classified as "light downhill."

it also depends on what you mean by rocky trails. porcupine rim would not be a good place for a hardtail.

so...let me answer your question with a question.....do you want to buy a full suspension bike?

if you want to justify buying a full suspension bicycle because you want full suspension but aren't sure you should get one, then yes, you need one to ride rocky trails, go buy it. if you don't want to buy a full suspension bike, then no, you don't need one, there are plenty of people riding rocky trails on hardtails.