Hm. This is possibly one of the most outrageous suggestions from a play who is not new to the game..Sure you put forth a good arguement, but for the most part the idea is crazy.
Most people have never stolen a c3. You gather c2`s as you get them and cluster them when you have 5. Takes a LOT of work to get 5 good c2`s, At least now with mostly a dead game, where good planets are few and far in between.
Secondly if viral couldent infect clusters, theres would be less planets for terrans and miners to take, because the virals would just plunder any they got..C4`s are extremely hard to make without exploring..Youd limit virals to basically 1 c4 that they explore and thats it..Or else capture 64 good land planets..im sure. Would limit viral combat to killing each other.
To sum it up, your idea is useless.