Just what is a sub job? Despite what most of the arguing is about, there's many uses for subjobs:
1) Specific Ability Sub
The point of such a subjob is for a certain ability useful to whatever quest or farming you migh tbe doing. Usually, the rest of the sub's abilities won't even be used, and the whole point of the sub is for its one or two abilities.
On a side note, ninja 2hr ghetto warp is sometimes used in a similar fashion.
Example(s): Tele-taxi WHMs with a BLM sub for Warp, /THF for Gilfinder, TH, Steal when farming.
2) Superpowered Sub
This is probably an awful title for this, and the reason is it doesn't come up much. The point of this sub is basically to give you a (sometimes unfair) advantage over your opponent. The below examples are pretty much the ONLY time I ever see these
Example(s): /BLM from lvl 1-7 to solo like crazy, /NIN in Ballista
3) Skill Sub
A normally less useful sub, this might be applied in skillup parties to aid in capping skill quicker. The best example is below, and I think that it should be a clear example.
Example(s): /SMN in a magic skill up party for MP boost
4) Mentor Sub
This is another rare one, and should only be used with extreme caution and discretion. I'm going to list an example first, then explain the type of situation this should be used in.
Example(s): Melee (mostly war)/WHM, Melee/RDM
Before you stop reading the rest of this post all together, remember what I said about extreme caution and discretion. RDM is a job that had great survivability. At low levels, Valkrum in particular, even as a sub, this survivability can be added to the party. The whole point behind such a sub is to cover mistakes others will make.
Take Valkrum as a case and point. There are tons of awful valkrum party threads here. Should your WHM decide their divine skill is more important than healing the tank, you can do it in emergencies. Should your party be melee heavy, you can enfeeble. If you happen to be monk, or some other non-ranged weapon class, dia could pull. This type of sub banks on others stupidity
Even if you play this perfectly, there are some risks involved. Experienced players might pass up inviting you; you're more likely to get into parties where it's necessary. There's also the danger that the people that you save won't learn, and they'll just speed on to qufim and wreak havoc there. However bad ro dangerous it may be, it is still an option that I have included here
5) Enhancement of main Job
Here's where the arguments come into play. These are the only types of subs that should be used in experience parties, with few exceptions (see /rdm in above example) There are two main functions for this type of sub
a) Enhancement to the main job - Strengthens your abilities
b) Compliment to the main job - Allows you to use your abilities better
The minority of possible job/subjob combinations fall into this category. Again, the two possibilities are strengthen your main job's abilities, or allow you to use them more often - almost like deciding between a Great Sword & a Dagger.
Since I'm a Dark Knight, I'll use the two most common DRK subs to illustrate this point. The general conception, though not shared by absolutely everyone, is to sub warrior until level 60(sub 30), then theif after that. /WAR falls into category a, /THF B.
The advantages /WAR has (considering only the enhancement of the DRK - i.e. provoke does not help a DRK [not counting party aid yet, i.e. trick partners]) are: natural STR increase, Berserk, War Cry, and Double Attack. Aggressor would help, but is left out of the comparison due to the level cap.
/THF follows a different route. you would recieve a natural DEX bonus (minor Acc up), and the two abilities sneak attack and trick attack.
Now to compare: All of the WAR abilities directly increase the DRK's attack strength, which is their main forte. This allows them to deal more damage per hit than before. The THF, on the other hand, allows for slightly higher, though arguably negligible accuracy increases, but true strength lies in the trick attack ability, which when used in conjunction with sneak attack, do both great damage AND protect the DRK from enemy aggro. Thus, the /WAR allows the darks to hit for more damage, while the THF allows the dark to safely hit the enemy.
But now which is more valuable? Pros and cons must be weighed. Which set of abilities will help you more? Which will help the party more? You may prefer to have a thf subbed so you can sneak attack an enemy, but if there is a thf in the party, a war sub would be more beneficial becasue then you can be a trick partner for them. What will do more damage? SA and a SA, or a Berserk and a SATA?
Oh but wait! Some nameless strategy guide has also suggested /SAM for its skillchaining abilities! Weaponskill do quite a bit of damage, and since DRKs are Damage Dealers, it's perfectly fine to consider this potential sub. However, many Scythe and Great Sword Weapon Skills, especially at the first part of the game, can be quite weak. In addition, all the above mentioned abilities are lost. To some, this might work fine as a sub, but in addition to what is gained, you also have to decide what is lost.
In conclusion, I've tried to categorize the possible sub combinations. I've also tried to emphasize why subs that correspond to your main job are important, and why its important to weigh all the factors heavily. Know your job, know its limitations, know how to improve it, and that knowledge can take you far.