I feel like the problem with this thread has a lot to do with what we think of when we think of pick up parties for some people, they think of those god awful things that are just happy to have positive exp. For some people that means oh this guy with a ridill and this other guy with a relic feel like partying, I don't really know them but lets party. I personally don't merit without a cor or brd anymore, ever.
The quality of the party has everything to do with which sub is most effective. Sometimes you have a great pick up party, and for many people if it doesn't look like it'll be good, they don't even join. I know the awful merit PTs exist but I am never a part of them, sure I'll merit with random crappy DDs and do 2-3X their damage but as long as there's a decent buffer and healer, maybe a voker(even with crap gear) to help with pulls, it turns out ok. Those are my concerns when joining pick up PTs, and I rely on /sam for merit PTs almost exclusively.
That said there's lots of events I use /nin for, in fact its what I use for the majority of endgame events I do(well computer is in the shop so can't play for a few weeks). I know sam main has a little different of a situation especially if you are only comparing /war to /nin as one of the main reasons to use /war is for berserk but if you take hits with berserk up you will take significantly more damage, as I understand this is actually why rukenwhatever decided to go into /drg testing, because he found himself often unable to use berserk due to how much more damage he would take and /drg with earring easily provides more benefits than /war if you can't afford to use berserk, and high jumps allow for more hasso use than even a berserkless /war.
For sam one of the main points I guess is about if you have to full time or nearly full time seigan for /war or /drg then /nin with full time hasso will almost certainly do more damage but the less you pull hate the more you can use hasso, the more the non-/nin subs catch up/pull ahead, so it all depends on you and the parties you join.
This is a bit of a different situation than that which drgs and drks look at with /sam because drks have drain abilities and drgs can lose hate when things go wrong. Also we don't sub sam just for hasso, it gives us 6 hit builds and meditate also, these things make it blow /nin out of the water damage wise for us, while still giving the safety of seigan at least, where sam mains actually have many situations where if you are trying not to sponge /nin actually provides the most damage at the same time as the best defenses, I don't think the issue is so much the effectiveness of seigan, which may or may not be used effectively but rather that if you are being forced to give up hasso too much you are better off with /nin so you can go back to using hasso and that if you are pulling hate a majority of time which you will if you tend to be the most pimp DD in the PT than /nin is certainly the best option, if you only occasionally pull hate than /war or /drg should be best
As I take it rappas question can sort of be rethought of as do you assume you will(/nin) or wont(/war or /drg) have hate the majority of the time when you join a pick up PT? with another option for "I don't know why I sub what I sub" and with confusion due to no explicit definition of what a pick up party is other than that its sort of unplanned and not with friends. Resulting in very different mp situations being thought about, some suggested set ups seemed to scream out to me "I NEED 10+ mp/tic" others screaming out "what's refresh". Because the more refresh your party has, the more damage your party can afford to take, the more you can use hasso and again you can end up best not subbing nin even if you pull hate the majority of the time, if you have enough refresh.
sorry for rambling on when I'm not even a sam but having no computer to game on leaves me very bored.