Decoys are an interesting idea, and others have brought them up before. Here are some thoughts on the matter, not saying for or against, but things to consider.
When comparing missiles and PD, one thing to remember is that (at least in every game I've played) the PD cannons don't ever seem to "miss". It's only a question of whether they do enough damage to take out enough missiles.
This would lead me to think that maybe one way to augment things in the missile's favor, would be to allow for some higher tech missiles. Perhaps after you reach level 5 in CSM, the addition of ECM levels could allow for "smart missiles". Smart missiles would act like CSMs except that there would be significant "-to hit" percentages. Thus, the SCSM 1 would be a CSM 5 with a 20% defense bonus. The SCSM 2 would be a CSM 5 with a 40% defense bonus, etc.
Another thought would be to add "decoy missiles" as a tech, linked to missile technology, and again. maybe to ECM. To make them effective, they would need two things (potentially).
1) Better speed than the normal CSM of the same level. Maybe +1 across the board.
2) Either make them a smaller component, or increase the rate of fire.
Since they won't cause damage, you don't want them to cost as much resource and space wise, as a normal missile. Otherwise, why not just use more missiles? The reason for the greater speed is so that they will soak off the PDC fire BEFORE the real (warheaded) missiles come through the zone.
I think that either implementation would be a neat addition to the game, but it's not so critical that I'd ask Malfador to delay work on the AI in order to do it.
John