Also further to what Niwashi said, above level 50 the quality of the gear you equip to them actually directly affects the likelihood of them bringing back rare items, the higher the item level of the gear they have the better the chances of them bringing back rare loot.

So don't just stick level 1 glamour gear on them (or worse, no gear at all other than an old weathered weapon you had lying around!), make sure their gear is as decent as possible if you want them to bring you back worthwhile things.