WHM/BLM
By far this is the most common subjob combination.

-At 34 you can warp which is very nice.
-At 40 you get the BLM passive trait Conserve MP
-At 58 you can escape which is extremely useful

This is a well balanced combination and you will never have to deal with people saying WTF?



WHM/SMN
This is less common but still very useful.

-At 50 you will get Auto-Refresh which is arguable better/worse than Conserve MP
-More MP in than BLM but without Conserve MP you will definately use more of it up

You will see this a decent amount in endgame situations where the MP bonus is very useful and you will almost always have a BLM around to to escape or warp you.


WHM/BRD
Some people have suggested this but...

-At 10 you get Resist Silence
-At 40 you get Resist Silence II
-At 50 you get Ballad but isn't it more useful to have Auto-Refresh and not have to sing it on yourself?

Now you can sing other songs to help your PT but keep this in mind. /BRD cannot use instruments. This has 2 effects... you can only sing one song and the songs you sing will be DRASTICALLY weaker than a BRDs song with instruments (especially the right instruments). Even though you have blink and stoneskin to keep you safe while buffing the PT you will have to run into harms way and waste time and potential resting to keep songs up. I suggest you steer clear of this.

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Really I think that's about it on useful subs for WHM. Some people sub RDM for fast cast but it's really not worth it especially considering the MP loss from not subbing BLM or SMN.