I can only give you my personal opinion. I've occasionally PvMed on my red Ninja, fully decked as she is my main char.. Been hunting for fame when bored, against Demons, Liches, Dragons and such.
I wouldn't include Ninjitsu on any PvM character. The Animal Forms are too restrictive(if you want HP Reg, go Beast Form.. If Stamina Reg, go Vamp), Deathstrike is a complete waste of mana and Mirror Images, well... Not worth the hassle. Evasion or just simply Parrying will help a lot more in avoiding damage. Even Attunement is worth more.
Being a Samurai offers a lot more and if you don't like that then go for Chivalry or Necro; Add SS and cast some usefull spells or don't, and use it for the Forms).
*NOTE* About Necro, you can just pile +Necro % items and use the Forms that way if you won't add SS.
Back to the point, Focus Attack in particular can't be used like LS or other moves you can chain instead of a normal attack.
On top of that, there are skills out there that for the 10 mana you spend on a Focus Attack will do a lot more and will be worth chaining.
Even some weapon special moves can prove far better for PvM.
More importantly you cannot equip a shield(!) for this move to work, nor a bow(?).
I can't remember but I think it will boost your DMG by 30%ish and you can be sure your weapon mods will go off 100%.. Or something like that. In PvP they always go off at 120.0 Ninjitsu.
There are other ways to get that 30% extra damage, even permanent like Beast Form or well, DI items, skills and such. Easy to reach damage cap really.
So your only real benefit is spells going off. Add about 15 damage depending on spell in other words(correct me if this number is wrong).
NOT worth it for PvM, 15 won't make a difference.
PS.
If you are so keen on getting every single extra damage point you can have, invest on Spellweave and use Immolating Weapon, holding a weapon with spell effect on its own. A lot better.