FOD inflicts Shaken. All bosses are immune to Shaken, and other creatures get Shaken frequently anyhow. Damage a character wearing a Fearsome robe and you're auto-shaken. So having a high DC on the shaken effect isn't too impressive, because it's not uncommon for things to be shaken without a save at all.
Nonetheless, it's true that in some common situations, a finisher which involves FOD can be considered to have an effective +2 DC, because you'll debuff the monster on the way towards executing the finisher.
That's because the healing moves are better when it matters. Domination moves have an intrinsic self-contradiction in their value: they are single-target debuffs, meaning they are most useful against powerful individual monsters... but the powerful individual monsters are red named bosses, which means they're immune to most Domination debuffs.
Domination Monks vs Bosses
Fist of Darkness shaken: immune
Touch of Despair healing-resist: vulnerable (but almost always irrelevant)
Pain Touch nauseate: immune
Falling Star blindness: immune
Freezing the Lifeblood paralysis: immune
Karmic Strike damage: vulnerable (and he could one-shot you with his next swing)
Back when monks were first released on Risia, I put in a suggestion that when a red-named creature is immune to a monk's debuff, it should instead suffer some kind of reduced penalty (dazzled instead of blinded, sickened instead of nauseated, reduced caster level instead of no casting, symbol-of-pain instead of paralyzed, etc). But until something like that is done, I see Domination as the weaker choice. Against bosses your debuffs are harmless, and against other monsters your group will probably kill them too fast for a debuff to be valuable.