Genuinely surprised this isn't already the case, they are functionally identical to Runes outside of the limit of 1 recharging at a time.

Please change :)

The api for evoker essences has been broken since the release, they apparently fixed that with 10.0.7 release, so we indeed are planning to take another look at it.

In the next release there will be a essence trigger, but note:

• Your intuition that essences are similar to runes is actually wrong. Essences are more similar to e.g. rogue's energy
• The api blizzard provides is still broken, the display will randomly be incorrect. That can't be helped. We have reported that as a bug, but well they haven't fixed their own essence display in 6 month, so clearly they don't care.