it's not really a bad idea, i agree with the others.

and the cap for the endgame tomes, when content is still relevant, is 450 (unless they change it for SB) so it's fixed. so having cumulative bonus (for the lower tomes or both lower and endgame tomes) wouldn't hurt much of the relevant gear progression and would also help newer players catchup in gear item level from the older/lower tomes so they can do the relevant content earlier = more players in endgame = faster filling pf's/rf/df/etc. and most people used to cap the 450 weekly top tomes fast anyway so it's not really a big deal