Gonna try to keep this short.
Basically, subtle blow caps at 50. Currently the innate traits at the level cap give NINs 27, MNKs 25, and DNCs 20. Subbing NIN will get you 15. Any sane NIN will merit subtle blow instead of the amazing -ton spell enhancements in group 1 so they'll have a base of 32.
Add in Rajas, that's +5.
A WHM with Auspice gives +10, and if I'm not mistaken, Orison Duckbills +2 will push it to +20. A Ninja can cast Myoshu: Ichi and get +10.
So with all that, it's easy to see that it's laughably easy to cap subtle blow. I think just that alone should be reason enough to raise the cap really, since MNK and NIN have a bunch of other gear on top of this that has no real use when it's so easy to cap the stat.
But a bigger problem comes when you consider the way monsters gain TP. The idea behind jobs like MNK and NIN is for them to be "low TP-feeding DPS". The concept falls flat when they're basically feeding the same amount of TP as the two-handers jobs- Monster TP has a static value added to each hit that connects, so faster-hitting jobs will be feeding more TP assuming no subtle blow.
tl;dr
1. Too easy to cap subtle blow on jobs that "specialize" in it, makes a bunch of gear useless
2. Even when you do it doesn't make said jobs feed less TP, it just equalizes them with two-handers