The ilevel gaps are bridged by making a second tome not capped, for exmaple you could get 230 gear for a while and 250 crafted, so with a combination of this, meeting any ilevel requirement is easy.
This is even more true right now, because of the fast queues currently for PvP and that gear is 235, ALL SLOTS, and that can get you in anything. (Dun Scaith the weekly 260 drop is 235)

I agree with having a carry over with.. the higher level tome (currently scripture) though.

The reason for the high level gaps have to do with the hardest runs to do, and they have drastically higher gear needed, so you get scripts and whatever to help meeting to go to it, then get good gear from that to continue on.