There was quite a bit of discussion this last weekend both when I was around and when I was not on whether or not it is important to keep your subjob leveled up. Some people seemed to think that it isn't as important to keep your subjob up and that it is completely and totally fine to let it slide by a few levels. I found a great post on this subject here which I am going to reproduce below. This post really does explain why it is important to have a fully leveled subjob.

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The problem is that an underleveled sub IS NOT a handicap. Support Jobs only enhance the main class, not detract so any level of support job you have is a benefit. Having a fully leveled support job doesn't mean that player is good at doing his job or even as a person. You severely limit yourself in options and horizones the mindset that having to have a fully leveled sub (and to a lesser extent, odd-job combinations but that's a whole different thing) is a must for every single player. I do agree that having your sub leveled for milestone levels is important but having 2 or 3 levels inbetween those (say between a level 26-29 for WAR sub) milestone levels won't matter.

FYI, I went to 50 with a level 12 WAR sub myself and I leveled much faster than most of the NA population because I had a good rep =b.
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ANY subjob is technically a bonus, so why wouldn't you want the best bonus you could possibly have? The following is a comparison of two Hume PLDs:

PLD50/WAR12__________________PLD50/WAR25
HP: 820______________________HP: 879
MP: 212______________________MP: 212
STR: 51______________________STR: 54
DEX: 41______________________DEX: 43
VIT: 52______________________VIT: 54
AGI: 35______________________AGI: 37
INT: 33______________________INT: 35
MND: 45______________________MND: 47
CHR: 46______________________CHR: 48

Essentially, Shouta, if you have leveled your WAR sub 13 more levels, your sub would have given you the following stat "bonuses":

HP +59
STR +3
DEX +2
VIT +2
AGI +2
INT +2
MND +2
CHR +2

People spend disgusting amounts of gil on equipment for smaller stat boosts than the ones you see here. Why wouldn't you take out the time to level your sub for these, rather than pay a LOT of money for the stats. Or, better yet, why wouldn't you do BOTH?

Further, a few levels in a sub DOES make a difference, so leveling only to milestone levels STILL lacks some benefits. For example, you are a PLD60/WAR25 according to your profile. While the stat calculator is not accurate to level 60, it is pretty good before that. Here are the "stat boosts" you would gain by leveling your WAR sub to 29, instead of 25, a mere 4 levels:

HP +17
STR +1
DEX +1
VIT +1
AGI +1

If there was a piece of equipment out there that offered these stats and magically took up no equipment slots, do you have any idea how much it would cost? Millions. Level 75s spend 10K experience points raising ONE attribute ONE point. Lacking 3 or 4 levels in a subjob is tantamount to being a level lower in your main compared to someone who has a fully leveled sub.

And by the way, one of the reasons you leveled faster than most of the NA population was because while you were leveling your main job non-stop, most of them were taking a break from it, leveling their subs.

And don't think mages can get away with this, either. Arguably, a mage with an underleveled sub is even worse. Greedy, by not leveling your SMN sub to 15 and staying at WHM30/SMN09, here are the "stat boosts" you are lacking by not having a /SMN15

HP +11
MP +28
DEX +1
AGI +1
INT +2
MND +2
CHR +2

Six extra levels of /SMN is like having an Astral Ring, Saintly Ring, and Eremite's Ring ingrained into your character's base stats. In fact, a /RDM15 will give you 6 MORE MP than a /SMN09!!!

Obviously, subjobs are only a bonus. However, when the standard is having a fully leveled sub, and your is more than 1 or 2 levels behind, then compared to the general playing population, your character IS gimped.

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Hopefully that should give everyone a general idea as to how important a leveled sub is for stats, let alone the invaluable Job Traits and Abilities.

Now, let's say this person you know gets to 22 WAR. Ask him if he would like to have +12HP and +1 to EVERY stat without even having to buy a new piece of armor. Then tell him that this is what he will get if he levels THF to 11 (instead of 7), along with the added bonus of Evasion Bonus. Because a difference of 4 subjob levels at this stage of the game means just that, a difference of HP, MP, and at least 1 of every stat along with the traits.