As stated by the others, I haven't had any problems with everything in my waterproof bag getting wet. In fact, I just did a tour that ended up being very wet, so I was packing damp clothes in my pannier a lot, separated in a dry-bag, and didn't end up with everything else getting wet. I keep my tent on my bike, not in a bag. When it's raining, I set up the tent and then put my panniers in it before unpacking stuff, so no issues with rain getting in there. You also don't end up getting a lot of rain going in them in the short time it takes to grab a few things out if you do open them. As mentioned by others, it just isn't an issue in practice. You can make a lot of theories sound good or bad on paper, but waterproof works. My only experience with non-waterproof panniers was riding with a friend who had them. He had covers, but his stuff still kept ending up a little damp when it rained...

Just to expand on the attachment system, Arkels do win that contest for me. I don't have any, but I have experience with them from my touring friend and they are Really easy and secure. But several newer attachment systems are pretty sweet these days. People have figured out some pretty hassle free, but solid mounting, systems.