You can summon 4 types of Support Unit in this Game, they are (examples)
And out of the 4 kinds only decoys will be destroyed, but out of the 5 classes only Monks have a 0 cooldown decoy.
I have no idea how much Life does each pets have but Mystic Ally can deal 40% dps and only cost 25 Spirit. The nearest by comparison to cost is Demon Hunter's Boar Companion, deals 38% dps and cost 10 Discipline with a 30 seconds cooldown. A Monk can gain about 100 Spirit per second while a Hunter can only only gain about 20 Discipline per second. With 100 Spirit you can make 4 summon, with 20 Discipline you can make 2 summon but each will be 30 seconds apart. In other words, Companion should deal 3-5 times more damage compare to Mystic Ally. If the cooldown in Companion is to prevent players from wasting their resource then a 1 second cast delay similar to Shadow Power is all it needs. There is no reason for this cooldown, it makes them WEAK, and useless in time of need. I'm not the designer so I have no idea why the cooldown was added in 1.0.5. Each decoy type skills need to deal more damage if a cooldown is stopping players from casting them.
Mirror Image is the funniest decoy type unit, it only last 7 seconds but it also have a 25% of your Life Pool, so it will die if it is hit. The skill have a 15 seconds cooldown, so personally I think it should work like Zombie Dogs, Companion, Mystic Ally, etc. once summon the unit stays until destroyed. This is the only decoy unit that have tooltip telling us how much Life it has, but it is also the only unit that DOES NOT deal damage by default. Zombie Dogs can deal 9% and there are 3 of them which adds up to 27%. Add 10% damage to these shouldn't hurt. Now, if you look at the video in the Game Guide, you will see that after casting "you move" away from the mobs, but this is a lie... The spell will "move you" into random direction away from your position of cast. If the skill is like the effect on http://us.battle.net/d3/en/item/the-grin-reaper where the copies are summoned by your side without moving the player will give us more control over the spell.
There are a few 120 seconds cooldown skills, and they are suppose to be the most powerful skills in the Game. But unfortunately some of these skills are decoy type summoned units. Decoy type units will be attacked and destroyed. IF they stayed alive at all time they can deal massive damage to nearby enemies.
3 Units, 60% damage, 15 second duration
3x 60% x 15 = 2700% weapon damage
5 Units, 20% damage, 20 second duration
5x 20% x 20 = 2000% weapon damage
Now, if you ever used these skills how long did these units last? Depending on the monster strength, I would say 3-8 seconds. So players are wasting 120 seconds of their time to summon a few pets to dance around and die before they can even deal damage, yes, these units sometime die before they can reach their target. Summoned units with a duration limit should work like turrets and stations, they should not take ANY damage after they become summoned.
Skills are not under used by players, they are under used by the designers. These kind of problems would not have existed if the staff team have tested each skill and their possibilities and potentials in different scenarios. Why give players a choice that will be removed after a few attempts. I'm pretty sure the team designed every skill and rune so there are massive numbers of build for players to choice from, but why am I not seeing them.
This is a bit off track, but I would like to add.
Wizard can only summon 1 Hydra, Demon Hunter can only summon 1 Sentry, but after 1.0.5 Demon Hunter can summon 2 Sentries with a 8 seconds cooldown in between.
The cost of 1 Hydra is 1.5 seconds with base Arcane Power Regeneration
deals 28% x3 = 84% for 15 seconds
The cost of 1 Sentry is 7.5 seconds with base Hatred Regeneration
deals 175% for 30 seconds
= 2625 per 15s, 5250 per 30s
Sentry cost 5x more resource, but deal 4x more damage, and the ability of having 2-3 Sentries on the field easily over come this cost difference. IF Hydra base cost is increased to 30 AP with a 5 seconds cooldown in between, but allow the players to summon 3 sets of hydra on the field, we have the following result
deals 28% x3 = 84% for 15 seconds
= 1260 +840 +420 = 2520%
and after 15 seconds = 3780% per 15s as you can keep summoning hydra before it ends
compare to sentries
= 2625 +1225 = 3850% at 15s, 5250-7875% per 15s (2-3 Sentries).
The number might look lower form Hydra but you can cast different Arcane spenders with Wizard while having all 3 active Hydras where as for a Demon Hunter it will be nearly impossible to keep up all 3 Sentries and keep using Hatred spenders without using other means of Hatred Regeneration.