Let's think about this for a moment. Use that big primate brain. 

You already have to be a citizen to Vote in MO. But, I digress. 

So how do you prove that you went through the process to get yourself verified that you're a proper citizen? 

You're going to want to carry some kind of document, aren't you? It's rather pointless to have a system you both can't verify. 

This is why folks are saying that your RealID won't work to vote. You'll require new documentation of some type to show you went through the process. Otherwise it's kinda pointless.