My opinion on MNK subs. Note that this is mainly theorycraft, as I've only got a level 15 MNK.
WHM, BLM, RDM, SMN, BRD - Don't make me hurt you... :evil: Unless soloing/questing/anything not involving gaining exp, these combos should be avoided at all costs.
WAR - Berserk and Double Attack are very nice. Also, being able to Provoke in some sort of emergency is helpful.
THF - Sneak Attack and Trick Attack are the only really useful things you get from this sub. Since SA is only a normal critical hit on a THF sub, and TA doesn't add any damage, I don't think it would add as much damage as a WAR or SAM sub.
PLD - No idea why anyone would ever want this in an exp party...
DRK - Several Attack Bonus traits and Souleater at level 60 make this sub somewhat useful. Not sure how Souleater works on a sub, but if it still works similar to how it works with DRK main, having it turned on with a MNK's high attack speed would mean an insane ammount of damage dealt very quickly. Unfortunately, this also means the MNK nearly dies from lost HP and gains a ton of hate, so it's only good if the mob's near death already.
BST - I suppose this could be useful, if you party near EM or weaker mobs, and have a lot of +CHR stuff, but I'd recommend against it...
RNG - MNK can't use much for ranged weapons, so subbing something that focuses only on ranged weapons would be somewhat pointless.
SAM - As others have said, after level 60 this is good for two Store TP traits and Meditate. Before 60, however, there's not much to get from subbing SAM.
NIN - Ninjutsu is the only thing you really gain from NIN subjob, as I don't believe the dual weild traits work for H2H weapons (I could be wrong). If you have hate issues, it might be worthwhile, but otherwise I'd avoid it.
DRG - A DRG's main point is the wyvern, and you can't use that with a DRG subjob. Jumps and an Accuracy Bonus trait are all you get from DRG sub, and I don't think that matches up to what other jobs give.
So, overall, I say only SAM, WAR, and maybe THF would be good subs for MNK.