I too use plastic bags and dry sacks, but the reason I put things in those bags is not for extra protection from the water while they are in the panniers, but for those moments when I have to take something out of the panniers while it's still raining.
Also, imagine the situation where it's raining and you have just set up your tent, standing in front of it (or entered the room) with panniers in your hands, and now: You either wipe off excessive water on the outside of each pannier and then throw it inside the tent because it is completely dry inside, or you open each pannier and take out one plastic bag after another and wipe every single one because there is water in the panniers.

I don't have Ortliebs, but would say that not having waterproof panniers is a major inconvenience rather than that having them is a convenience. Of course, it is all a matter of personal preference, types of rides and weather we ride in, so what is important is to get what does the job well and what does not put us in inconvenient situations.