It's an interesting idea (I would love to be able to use dedicated EW specialist warships in my fleets!), but how would it be balanced?
Currently all of the jammers work on a per-ship basis - a ship either has a jammer of a given type, or it does not, so we need to define how this would work in a fleet setting. I can think of a couple of ways this could go and each has its problems:
- As in the OP, make an area jammer component that has less (e.g., 50%) EW effectiveness. The big problem with this is that it will probably be ineffective in most cases, since the enemy would need only half the ECCM level as usual which is easy to research and deploy.
- An area jammer component which is normal effectiveness but larger and more expensive. The problem here is scaling - if, for example, an area jammer is 10x the size and cost, then a fleet of 11 ships or more will gain a net benefit and the area jammer would be strictly superior (unless and until the EW ship is blown up). On the other hand, if we make it some kind of ridiculously expensive system, then smaller fleets don't really benefit. Either way, we have the issue that the mechanic makes a particular kind of fleet more optimal which is not really the kind of game Aurora wants to be.
If we have to choose one I would probably prefer the latter. In principle, favoring larger fleets is not a huge issue since you always want a bigger fleet for a major battle, but in practice we always need small fleets for a variety of tasks such as system defense/garrison roles, so there is a decision to make about how to design these classes - do we build them without jammers, to get the most benefit from the area jammers when our DDs or CLs participate with a larger fleet? If we build our CLs and DDs with jammers, then these would be redundant in a larger fleet but our small patrol groups will be more capable. Or we could have two different classes of each type specialized for each role, which may require more shipyards and complicate fleet compositions but in theory means we get the best of all worlds.
One mechanics note: it is probably better not to make area jammers tied to a specific fleet but rather to just affect all valid targets in range of that jammer. Otherwise we run into a silly case where one fleet performs differently than two fleets at the same location, which should not happen in this case. I would also suggest that the area jammer should increase in size and cost as its capability increases (in addition to the cost increase from higher ECM level, of course), to account for the fact that fleets tend to get bigger as the game goes on and thus to keep the cost benefit of the area jammers reasonable.