A question and an opinion...
First, does the 10% these abilities knock off the total mana pool effect the damage that mana abilities do? I suspect it does, but I have not attempted any testing to verify. Wondering if anyone has looked into it.
IF SO, then you have an ability that takes 2 slots (one on each bar), costs 10% of your mana pool AND negatively effects your baseline damage.
Since these abilities are generally NOT tied to a weapon, I am not sure why they turn off if you don't have them on both bars. If they effect both baseline damage and permanently deplete the resource pool it seems like overkill to have them also take a valuable slot on each bar to remain active.
Frankly, I would rather they were just on extended timers like several other abilities out there including several of the damage shields. These abilities don't shut off because you switched bars, you just have to periodically switch back and turn them back on. I would much rather the toggles behave this way. It would open up build diversity.