Um, the fact that it gives him 6 points per training manual is kind of the point of this skill, as his High Boost skills are expensive and you can only get training manuals at random from bosses, making this skill not as powerful as you may think. After all, the challenge here for him would be to gain the training manuals to help him out in the first place. I'm more or less at the end of the game, and I have close to 40 of those training manuals, and I haven't used any of them.
And the simple fact is that even with those skills maxed out, he's still not that good due to the fact that he has no attack spells of his own outside of subclasses, so the fact that gems are more potent on him than on anyone else helps him with that. And while I'm aware that he's primarily meant to tank and support, bear in mind that none of the offensive subclass spells are that strong to begin with, so there's no real harm in having him gain better stats than everyone else if he doesn't have any truly powerful spells to use them on. Raw stat power alone do not break the game, after all.
Plus, I stated before that at max level, 1 training manual equals 1 SLv in one of his High Boost skills, so you would need 5 of them to max out HP, TP, ATK, DEF, MAG, MND, SPD, EVA, ACC, Elemental Affinity, and Status Resistance, which leads to 55 Training manuals on top of the twenty that you would need to power the skill itself(the reason that I don't count MP is due to it's lower cost, so it should be covered by normal skill points), which is far more than you would expect to see in a single playthrough of the game. That means that you can't cover every High Boost with this skill, because there just aren't enough training manuals to go around to do so.
The only thing that this skill would really make him is a mid to late game bloomer, and that's only if you can get enough items to power him up in the first place. Random chance and all that rot.