Howie,
Rolling the top of the bag is standard feature in many (most really) dry bags. It's the roll that makes a waterproof seal (imagine the route water has to take to enter the bag). Ortlib's yacht bags use the roll closure as their handle. When the bag is packed full it can be a bit of a pain to get them to roll/close properly, but generally they are going to be the best panniers you have ever had.
--A