Instead of "evolving", the easiest way to make EPs (and XPs) useful to support crusaders (that is, all crusaders except one in any formation...) is to tie them to formation skills. This would mean a huge increase in DPS (and gold) though, so it would require a steeper increase in monster HP (so it impacts only end game players, those with tons of EPs, but not newbies). A 0.5-1% increase in the signature skill for every EP seems pretty fair to me, if difficulty increases faster.