there is a simple reason it doesn't do it automatically, because you'll notice that most of the large queues are only slightly over 100 long, this is because once a queue reaches 100 people stop entering it because their AEQ setting wont auto-queue... if everyone could auto-enter queues up to 300 long, then when a queue reached 100 it would keep growing, it would get up to 300...
then someone would want to be able to join queues longer than 300 say 1000, so then the problem just continues to get worse and worse until you end up with a situation like on the ed2k network, where most clients limit the maximum queues to 2000... and as a result it takes quite a while to get to the front of the queue!
personally i think the limit of 100 is too long, a queue of 100 takes ages to reach the front of, a limit of 10 or 20 would have been a better idea... oh well