I tend to grab Artemis IV more often than not when using a LRM 15 or 20, but it is still very much an expendable piece of equipment if I am running low on mass for more important things
An LRM15+Artemis IV is, on average, just about as effective as an LRM20, but lighter, cooler, and more ammo efficient.
-LRM15+Artemis IV: 8 tons, 5 heat, 8 shots/ton, 11.5 average missile hits per shot (12 hit on most likely roll), generating on average 92 hits per ton of ammo.
LRM20: 10 tons, 6 heat, 6 shots/ton, 12.7 average missile hits per shot (12 hit on most likely roll), generating on average 76 hits per ton of ammo.
So the LRM15+Artemis is saving you 2 tons, a point of heat, and probably a ton of ammo when you get one instead of an LRM20, and is going to do about the same damage in the long run. Yeah, ECM hoses you, but that is always a risk with the advanced electronic options.