Actually 1) they have become more vocal in their opposition lately and 2) regarding the "impression of a legitimate opposition" and "can be co-opted when necessary", this applies equally if not more to parties like Just Russia, which were actually invented by the Kremlin to give an appearance of competition (and I would argue that if Medvedev starts his own party the same should be said of that) or LDPR which votes with UR but siphons off the extremist crazies during elections giving UR an air of sanity in the midst of madness.  

The Communist Party is at least a genuine party seeking to provide a center left alternative to the center right UR.  They are a hell of a lot more of a "legitimate" opposition than the disorganized micro-parties which have little to no popular support but are most vocal in their opposition to the Kremlin.  

"Pretending that you already know the answer when you don't is not actually very helpful." ~Migeru.