It might encourage more sieging if we could siege once in each category. instead of just once ... As an example, if i could i'd siege in each of the 1,3,7,14 & 21 day categories I would. I dont believe this would generate mass siege queueness, not with the number of castles nowdays.
I did consider this during the siege update, and seeing as only the 1 day timeout series is full (with few queues), I may change the rules as you suggest.
I think the new queues would fill up quicker if the older ones had been emptied, I left all of them by choice, but clearly many others have not, for whatever reason.