Is it between the rear (gear) block and the spokes?
If so, it's to prevent a badly adjusted rear gear mech from throwing the chain off the top end of the block and down into the gap and against the spokes, where it chews up enough stuff to frequently require a wheel rebuild.
Obviously, if everything is set up right (ie: the gear mech travel stops) this can not happen, so the plastic disk is not needed. So, if you are confident everything is set up well, it can come off (by removing the block first).
Although your description of these things is accurate, there have been a couple of occasions on my mtb, where picking up a stick has caused the same thing to happen, and if I could have overcome my vanity, that plastic disk would have prevented quite a bit of hassle. But on a road bike? I don't think so.