The problem is really just the tradeoff involved in taking it is way too low.
If it took a large amount of resources to mount so that it was an actual choice in mounting it or not, or if the ECM chassis were significantly worse in combat situations then it wouldn't be so silly strong.
I think the issue is not that the tradeoff is too low, but that it fluctuates based on what you want to do. The problem is that the most significant cost of ECM is being forced to use a specific chassis, and so if you wanted a build suited to that chassis already the cost is nil and if you wanted a build unsuited to that chassis the cost is exorbitantly high. That's the issue. The cost isn't always high or always low, it can be either.
Piranha Games seems to be in a position where they want to make a Battletech game to please the fans, but also realize they need to make a game
game to please anyone with common sense. This became evident to me when I realized just how much some people care about role playing in MWO. I think given time Piranha is both capable and willing to rebalance Battletech conventions.