Fair enough with the HP effect, i think you are right, it would be very hard to make it not so useless considering how much there would be hitting it, i think your on the money with that, i think a perfect middle ground would be just a timer, does not have to be a short one, maybe 100 seconds or something, and then it does the same animation of disarming its self as when you place a new one.
Or maybe you could only place one down at a time while a single one is active, which could be slightly less time, maybe 30 seconds or 50, and then some kind of indication on the HUD were the ammo is when its about to run out and disarm, so there unlimited in use, but due to them being placed down and stay there for a set amount of time, the player has to be smart about were to place it, as it will be there till the time is up, then can place another down and each one would be important choice to the player, and its current function right now could be a cheat in the console, that way the function is not removed and peps can have fun with it how they see fit, again, just spit balling idea's, if you are happy with it as is, then as is, it shall be.