If we're talking about ninjitsu without hiding and stealth, I'd highly recommend going with bushido instead. Lightning strike criticals along with confidence to heal is much more useful than ninjitsu (without hiding and stealth) in PvP. To hedge against being dismounted, bushido allows you to ride a lesser hiryu, which can help get some heat off you if you're being ganked, and allows you to remount instantly (without the delay of mounting an ethy).
Now, that's just a straight up comparison of ninjitsu vs. bushido for PvP. If we're talking about ninjitsu + stealth & hiding, that's something else entirely. In combination with stealth and hiding, ninjitsu is awesome. Basically, it allows you to only fight when you have the advantage. You can position yourself while hidden, always get the first attack and first off DP shuriken at the same time you shoot your bow. If you no longer have the advantage, you can buy time with mirror image, get away with animal form, or vanish with smoke bombs. But for straight up PvP where you don't want to spend a big part of the time hidden, ninjitsu is less appealing.
I actually use both templates in PvP. For straight up PvP, field fights, etc., I go with bushido, fencing and meditation (along with archery, tactics, healing, anatomy). The template packs a big offensive punch, especially after you disarm and bleed people, you've got enough mana to mortal, AI or moving shot them to death. On the defensive side, healing and confidence along with potions can give you back almost 100hp in bursts if timed correctly. As has always been the case with archers, knowing how to move is key. Definitely use a lesser hiryu, and set a key to remount (in part so you can do it quickly, and in part so you don't attack your own pet accidentally), and learn to time your heals so you can stay alive for 4-5 seconds before you can remount.
If I know I'm going into a situation where I'm outnumbered, or just for a change of pace, I'll go with the ninjitsu, hiding, stealth (I also use archery, tactics, healing and anatomy, though not exactly the same values). As described above, you can wait till you have the advantage, and put a real hurt on before people know what hit them. That said, I think this is a cheesy template to use when you're fighting even numbers. Even your friends won't want you to be hidden 90% of the time if you're in a big field fight; it just seems a little cowardly. But it can be tremendous fun if you're outnumbered. I've had quite a few nights were I stumbled onto a big fight I had no stake in (a free-for-all at a harrower is tons of fun), and just moved around and picked off all comers. It's not that hard to rack up 30-40 kills , and on occasion I've had twice that many in a single harrower. And when you don't have the upper hand, just move on away and wait for things to develop or find another target.
BTW, I'm in agreement with those who think that, without 4/6 casting, chivalry is pretty useless in PvP. I guess, a minimum of FC3 could make it useful.