Well, I would exactly call myself an expert on this, but I'd say that in a PT, a PLD should definitely have WAR subbed. And I say that solely for Provoke. I know it seems like a waste to some to have a sub solely for one ability, and not for stat bonuses and such, but with PLDs, it seems to be necessary to me. When soloing, of course, anything goes. Whatever works for you, go for it.
On the subject of underlevled subjobs, I have to say I find it lazy to not level them. The only time my subjob has fallen behind at all is now, with my 30THF/14MNK. And that's because now I have NIN, and I'm leveling that to be my new sub. Now I admit, sometimes the difference isn't that great from a level or two, and in that case, it's okay not to level it. But the way I see it, that's just more levels you have to gain next time it needs leveling. What really annoys me, though, is when I see something like BLM30/WHM5. I see this all the time. These people would be so much more useful to their PT if they leveled their subjob those 10 extra levels. I have a friend who has that job combo, and when I told him to get off his lazy ass and level his WHM, he said "Why? I still get invited quickly without it." How arrogant is that? I understand that people are desperate to get a PT together and start fighting as quickly as possible, but we should send a message to people like this by not inviting them and telling them that this is why we're not inviting them. I know that at higher levels this is already done, and we need to start doing it at the lower levels, too.
Okay, now for stupid subs. I don't notice this on people as often, but I've seen my fair share. For instance, I saw two WARs in Yhoator Jungle who were subbing BLM. It was leveled and everything, but BLM? You're not a DRK yet, guys, and this doesn't work. One of them even said to the other something like "Finally another WAR/BLM! Aren't we cool?" or something like that. I eventually blacklisted that guy, though for different reasons (not only was he a complete n00b, but he was an annoying jerk, too). And another time when I was PTing in Yhoator Jungle, our WHM left, and we found a new one after a few minutes. She was a WHM/MNK, and when the fighting started, she used a Meat Mithkabob and started spamming Boost as much as me, moving up to the front lines and meleeing with the rest of us. She managed to heal us okay for that battle, but I think that was more because it was a weak mob. After that battle, our PT leader asked her not to melee, as it made it harder for the tanks to keep the hate off of her. She responded with something like "Please let me play my way, I'm a tough WHM." At this point I told everyone that I had to go and left.
Anyway, my point is, know your role in a PT, and choose your sub based on that. And keep it leveled. Please.