Ok, let's see here...
Subtle Blow is simply reverse Store TP for the mob. +1 subtle blow = 1% reduction in mob TP gain. From a fast 2-weapon user, that TP gain is 5+3=8 per hit, or so. +50 subtle blow would cut that in half, to 4 per hit.
Now, as to how much good subtle blow will do for a THF... About the only good subtle blow gear for THF is the Dragon set. Dragon set adds up to +20 subtle blow. If you're already at a significant evasion rate (cap = 85%), it certainly won't hurt. If a dagger gives 4 TP per hit, full dragon reduces mob TP gain to 5.6 per hit. Depending on what you're fighting, might be worth it, might not.
Pln. set gives... I've heard +5 and +10, I'd assume +5. Either way, it's nearly useless, only good for the other stats.
NIN will not hit 50% without Monk's Roll or ridiculous gear. Base 25%. Merits add 5, Rajas adds another 5 (35%). Weapons add 2 (37%). At this point, only way to hit 50% is with lolSash and Hades Sainti +1... one's the rarest item in the game, and the other isn't a katana.
From what I've heard, lucky 11 Monk's Roll with a Monk in the party will add 25% subtle blow, which makes capping subtle blow very easy for MNK and NIN. (btw, NIN MNK MNK BRD COR RDM with Monk's Roll = nill TP for mobs)