Short answer: I agree, and have considered buffing it for this stage of the game. However, I know how effective it will be at later stages of the game.
Long Answer: Rekindle is the premiere one turn zero cost mana regen. Boost Effect stat has a chance to trigger an additional time when passing 100%. Max 300% Boost Effect guarantees triple proc, so it has pretty good scaling percentages (no Boost Effect pathway as of yet!). Combined with the Recharge stat, which boosts all mana gain by (+X%), you can readily generate a large amount of mana. At 160% Boost Effect and 300% Recharge it would return 90 mana, and at 260% Boost Effect and 200% Recharge it would return 80 - making it the premiere mana regen option for some builds that want heavy investment in both of those stats.
Unfortunately, all of that is theoretical right now, and I understand that the move feel like a let down as of right now. I've considered disabling it as a reward but decided against it. Moreso, it speaks to the problems with the current trainer system - Not being able to preview what you're unlocking isn't fun!
I will record this feedback and consider how to approach a solution that fits.