Retainers can gather hides, and yes, we can also get hides from killing things, however I do not believe that to be sufficient enough to prevent players from having a Shepard for the purpose of gathering off of monsters. Retainers gather materials that botanist, miner, and fisher can gather, however we are still given the aforementioned classes so that we can gather materials on our own. You can also get Iron Ore from killing Coblyn type enemies, but that is also something that is obtainable via gathering.