Well the best GM explanation I've heard is that its so powerful (nearly) everyone plays it, even non-refess.
Here's a solution/idea:
If you have at least 2 more [Refess Sphere] than points in any other sphere, get +2 SP. Otherwise, one unit gets +10 Hit Points/+10 Max Hit Points.
Or alternatively (albeit maybe harder to program):
If you have at least 15 Refess card in your file, graveyard and in play, you get +2 SP. Otherwise, one unit gets +10 Hit Points/+10 Max Hit Points.
Or even more alternatively:
If you have more Refess sphere points than any other, get +2 SP.
Or alternatively again:
[Open Skill] 
Get +2 SP.
But in all honestly, right now Refess is getting pwn't due to lack of SP generation. We have the most expensive units. And I don't just mean "Big Refess". I mean even our "small" creatures are more expensive for the ATK/AGI/RNG (which are the only 3 stats that REALLY matter the most in this game; HP don't mean much when ATK is clearly higher (see: Sphinx) and DEF is useless since Le Beau forced the EX packs and a good portion of set 2 to focus on anti-DEF).
Kesaran, while the cutest, is the worst SP generator. It has more HP than the Owl, but the Owl has the advantage of requiring only 1 SP to get +1 SP. The excavating magic archer may be level 2, but she can do some attack if she needs to and is once again a 1 SP for +1 SP trade off. Getting those precious 2 SP is just too difficult usually, as more often than naught you find yourself reviving your kesaran and only having 1 SP left...not enough for blessing. We do have the Angel, but unfortunately Verlaat is too rare for most Iczers and the Time Reader is quite fragile. Plus she is a character and in the end you only get 1 SP every 3 turns (except in eternal morning, which is more like "occasional morning").
Refess requires the most SP though as it also has the most abilities that require SP. Shield of the Heavens, Verlaat's Sword, Bardia, Seraph, Time Reader...although some other spheres have cards with effects too, they are usually superior to refess' in both cost and effect (for example, Shield of the Heavens can lock down an opponent, but Bander's effect not only stops the shield, but can wipe a board clean. Assassin survives after he assassinations and its optional, and I don't even want to get into why Seraph suffers without splashing Falkow or Gowen for more SP).
Finally, we have the worst of the SP generating Grimoires. We can all agree that the Falkow grimoire is probably the best (even if its only a CHANCE of an extra), but I almost NEVER see the Refess grimoire actually DO anything. Usually a unit is already dead or almost dead, and those extra 10 HP don't mean jack-squat. In fact, only once in all my time playing Refess and Big Refess have I ever seen this card's healing effects do ANYTHING useful. If it was like Cure, it would be slightly more useful, but not broken. It would be broken if it were cure for everyone. I think I'd prefer to see it as a temporary raise shield, even if it lowered AGI or something. *shrugs* just some food for thought.
A lot of Gowen players complain about their Grimoire, but I'll tell you right now Refess would LOVE that extra +1 AGI.
I'll also say that unless we see some errata in this game, it is doomed. Even if its a inconsequential errata (like making Magic Doll's SP be 2 or increasing Armed Citizen's attack to 5), it would show the players that the game is not only tested, but has the ability to change for the better given the environment.EVERY CARD SHOULD BE PLAYABLE.
Some cards just more playable than others.
By that I simply mean that cards like Sphinx and Ancient Zombie Lord, while weak to the point of unplayable, have enough combo potential to allow in as they are. But cards like "Will-o-Wisp" are just disgracefully bad. Its outclassed in effect (Fairy Dance), Soul Skill (Verlaat; Witch; Seraph), Level (all the other great spirits), base stats (any other level 1 except armed citizens) and even as a healing alternative (Recovery Powder, Cure, Angel, Priestess). The card is horrible and we need it to be changed to make it worthy of play. Even if it isn't a power player.
The worst thing that happens: we find our experiment was wrong and the card becomes too strong. We change it back or scale it down.