Counterstance is actually incredibly useful. I'll "Do it" for Alaik. The Ability that while creating a defense that allows you to take 0 Damage and in turn INFLICT the enemy with some pretty decent damage, drastically increasing your DPS as ~80% of the Enemys attacks become your attacks.

Yes Utsusemi Is amazing, But Alaik is right, Its not (RIGHT NOW! Not Post-Abyssea!) the same age where MP Was Gold and everyone needed to Keep up shadows/etc. Counterstance is an amazing tool for jobs that do-not rely on Evasion. Key words there. (Jobs that don't rely on Evasion)

That being said, on THF, the Ability is far less useful than on MNK, as THF have super-high EVA, So they are unlikely to be hit more than ~20% of the time (I think thats EVA cap?). Meaning you'll counter far less attacks, It'll be less on your DPS than a MNK would gain from it, by -alot-. But it's still there however.

When it comes down to it i think /NIN will win until THF Gets Dual Wield III or higher, After that its really a toss up depending on what your fighting, and where it is. If you ever do Inside abyssea content 95+ when THF is likely to get Dual Wield III, /MNK will outperform /NIN in everyway. Your WHM will never run out of MP, and baring the mob you're fighting, you should never be one-shotted either. /MNK's "Defensive" Ability doesn't rely on reducing your DPS (Shadow Casting) and instead only servers to INCREASE your DPS.

Then Again, NIN/MNK would probably be better than NIN/anything if you're Duoing a mob with a WHM. You won't need the extra Survivability from /DNC, and NIN's have less EVA than THF, while they can still cap EVA on a lot of mobs, They fall behind on some bigger game. making /MNK+Counter-stance more viable. Assuming the mob doesn't have a TP move that could potentially 1-shot you (not many do, Mind you).

Is really is complicated, but TL;DR I think right now THF/NIN is winning, If/When THF gets Dual wield III /MNK will be a better Subjob for Inside-Abyssea Activities (when duoing with a WHM, or tanking with WHM in group). When we move out of Abyssea, Not so much. (which will happen).