Agreed, the shoulder straps are very useful for carrying your panniers off the bike. And they allow the panniers to be PROPERLY closed thus rendering them very waterproof to the point where I never bother wrapping stuff in plastic bags as the panniers won't leak - ever.
Plastic bags aren't for the panniers leaking, they're for organising stuff, not having to bring dirty panniers into your tent and - where applicable - preventing the stuff from leaking on other stuff.
I keep my gadgets, wires, medication, wash things, clean clothes, food etc in dry-bags and not-quite-dry-bags
[1], but my down sleeping bag goes straight into a Back-Roller with just its non-waterproof stuff sack for protection. It's never got wet. I put dry clothing in a dry-bag to share a pannier with dirty clothing and/or a wet tent and seat cover.
As for shoulder straps, if you want to use them for carrying the bag on your shoulder, the best place for them to be while riding along is inside the bag where they won't get wet or covered in dust. That (along with having to re-adjust the length) is why I don't like the Ortlieb approach of using them to close the bag, or for that matter the Radical/Brompton approach of not having the strap on detachable buckles/hooks.
Of course, sometimes the shoulder strap is the only way to waterproofly close a lightly-loaded Front/Back-Roller, and therefore necessary.
[1] Alpkit Apollo: Silicone cordura with un-taped seams and a drawstring closure in a variety of sizes and bright colours. Great for organising your things, and it'll *mostly* contain a shampoo leak.
