Well, here's the difference.
If I'm going out on a trail it usually isn't a day trip. It's 3-5 day's. That means a cooler, cooking stuff, tent, sleeping bags, clothes, tools, spare parts, extra fluids, water, gas, etc... Even with the rear seat out, I'm short on room. Now add a second person and it gets tight.
I don't like piling gear higher then the wheel wells and it has to all be strapped down. Putting stuff on the rack is a last resort that I don't like using since it causes a whole lot of body roll with gear up there.
This is what I'd want a very low profile setup and be somewhat waterproof. Having decent sound and storage space makes it a bit more difficult.
(I've got my amp in the same location, but on the driver side.)