well... ahmm

1) strange turrets ai - they are not see hostile stay close from back

2)they are shoot to biggest enemy around->elite->usual enemy

3)Low "tower" rotation speed

4) low dps

I think it would be best to let the player pick up the turrets, but if they are destroyed they can't be resummoned, lore wise that is nice, and until now i have never needed all three turrets.

It would also make the player think strategy having a limited amount of turrets, but let the heavy class have more than 3 grenades.

Those are two different issues so they shouldn't be treated at the same time.

The despawn is to allow to summon new turrets. I thought it was the main issue here.

The duration of a turret is something I put under the notion of "balance" and I believe should be treated later, when all the mecanisms are implemented and understood.

I can agree than an unlimited duration is too long. Then any reasonable max duration would prevent abuse as the character is too slow to efficiently pull all the map to the turrets.

But a duration equal to three time the cooldown in order to always be able to put a turret can be too short.

And if the duration is set like this, the only remaining option to balance is to change its DPS.

So, as I said, not relying on the duration to be able to set a new turret is the best option as it leaves all options available (duration, dps...) to balance the turret as efficiently as possible.

I agree that the turret system is weird at the moment. If you're allowed three turrets at a time, then a "killbox" scenario should be fine, so long as the other classes are strong enough to compensate, or the other skills are weaker to compensate. If lining up three turrets to mow everything down is not something they want to happen, why allow three turrets at all? Kiting guys all over the map to make use of your turrets sounds like no fun.  But "saving" all your turrets for that one big room of guys and dreadnoughts doesn't sound like a great use of the mechanic either. 

And if the turrets you set down remain in the same place until destroyed, you will usually use them in a situation where the turret is easily overwhelmed and destroyed because you use it in a room chock full of guys, or they will mostly be useless over the course of a map because you use it in a place where it can help you kill 5 or 6 guys and then the turret stays put.

If we don't want a killbox, what about limiting it to one turret, that stays where it is when you place it, but can be re-placed wherever you feel like it after say a 30 second cool down, whether it's been destroyed or not or something?

It would be arduous to put down when you're only fighting 4 or so guys, but important in use and placement when you're in a room of bad guys, maybe making use of a choke point or something. Another idea is that maybe the turret could be a mobile turret that moves with you?

EDIT

At the risk of going on and on (too late!) The turret system in Diablo III is an interesting comparison, allowing you to place three turrets, (I think up to 5 with the right gear/skill choices). Placing each new turret can be done at any time and simply despawns the oldest turret in play. The Demon Hunter still uses their own skills and the turrets are a supporting element. The difference is that the Demon Hunter is a squishy and very quick, agile character who can dart away from trouble and use their abilities while simultaneously  the turrets do their thing. Clearly the crusader is not this type of character. Just wondering if there's something in that.

It would leave more options to balance the turrets and make them interract with various mecanisms this way.

Lets hope the devs will listen :P