Helicopter Engine IR Suppression Systems

I am slight confused about the use and effectiveness of IR suppression systems on helicopter engine exhausts. There seems to be no consistency in their use – some do and other don’t. For example – in the UK context the Lynx has suppressed engine exhausts but no other helicopters do, even in hot combat zones. The US military uses them on Blackhawks, Apache, Kiowa Warriors, Venoms, and Vipers but not on Super Stallions, Sea Knights, Chinooks and Hueys. :confused:
Is there some form of payoff in terms of effectiveness against modern IR MANPADS and their adverse impact on weight, drag and costs? Is it too expensive to design systems that have limited effectiveness in the modern combat zone to make it cost effective.

The other question that links into reduced IR signature is the use of the older ALQ-144 disco light IR jammer. I was under the impression that this was no longer considered effective. However, apart from the Merlin, I have not seen a combat helio with any other type of jammer. Wasn’t the DIRCM supposed to replacing these omni-directional units? :confused:

Can anyone enlighten me about the current state of play in counter IR missile technology in the modern combat chopper?