A majority of the 15-mins are lackluster. Warrior's Mighty Strikes really isn't that useful when you consider 2/3 of the Combo Finishers are based around "increasing critical rating" so a majority of the time you will land a Critical hit. So there is planning when you use the Warrior 2hr, you only have 15 seconds to make use of it and you might be able to increase your dmg by 300-600 during that time but honestly the damage increase is just as much as if you comboed and got the critical hit from "critical hit rate increase" combo.
Warrior's have two ways to think about it, they could either use Mighty Strikes and use it for the whole combo, or wait it out and use it at the end of a combo. If they use it at the start they risk missing an attack causing the 15 min to be wasted or forcing them to spend more TP to make use of the 15 min.
I'd say out of all of them PLD/BRD/WHM got the best out of it. Even look at MNK 2hr, increase auto-attack rate, so pretty much increase TP gains while also increasing mobs TP gains during that time, it's a double edged sword that may allow you to pull off combos faster but also allow the mob to use more of it's moves based on when you use it in combat. Plus you are still limited by your combo recast so even if you are building TP faster what's the point if you have nothing to spend it on.
Then there's DRG/BLM which are pretty much instant damage moves. Perhaps a way they could fix those 2hrs is if they made it where they could be used in any finished Combo to change it's effect. So say DRG's 3 finishers could be
After:
Disembowel=Decreases elemental resist of 15min and increases it's magic accuracy towards target
Rains of Talon=Increases damage and Raidus of 15 min
Doom Spike=Increases potency of paralysis
Chaos Thrust=Increases Magical crit rate of 15 min
Basically when you perform said combo and use the end of it, you would have a window to gain one of those effect based on which one you used last. So if you did the Chaos Thrust combo then you would be able to increase the chance of the 15min doing critical dmg.
On the BLM end it would be a little tougher since they only have 2 combos and it's already done with the Thunder combo but perhaps something like this could work.
As it stands now we have
Burst=does more dmg when you have less HP(if used during combo)
So the other two could be
Freeze=does more dmg when you have more HP but increases enmity gains
So the Freeze/Burst Combo could do maybe 50% more then burst by itself but it would be risky since it would grant you more enmity then if you were to perform 3 Burst at 100% dmg each rather then 1 Burst at 150% but of course since you can only use Burst every 15mins it may be considered worth it.
For Flare it could be a combo which "increases damage based on amount of DoTs on you with no change in enmity gains" So that combo could make it where a BLM may carry some poisons to injure themselves to add to whatever other DoTs they may be inflicted with to increase dmg.
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Perhaps it could work, but time will tell how they might change it if they do at all but of course XI still has this problems with 2hrs and it's taken them years to change it lol. So hopefully they'll be able to make adjustments to them quick so that they can all find a way to be useful when the situation calls for it.