It's high maintenance to have to wash it every night. Even if you did wash your cover and cushion every night that only reduces the chances of a leak. It doesn't prevent it. You don't want to call it a flaw? Fine. Whatever you call it, there is an inherent problem with the design. Given their track record over an extended period of time, they have no excuse to not improve their cover. Why don't the add a lightweight breathable membrane to the cover to eliminate dirt particles from reaching the cushion?
I'm giving them the benefit of the doubt that the problem is dirt. Otherwise, it's design and/or manufacturing issue. Regardless, it is a reliability issue.