This is silly..
First of all.. what does Rapid Fill do? It keeps guests from "stealing" coke. Since it does not allow guests to steal cups.. the only thing that could possibly be stolen is actual syrup. Now a few points...
1- In order to "steal" the coke syrup, the guests have to be staying at a premium priced "value" resort. These are the guests that are paying extra to stay on property. Since anyone staying on property is also going to be paying to get into the parks the people that could be "stealing" some coke syrup are already high paying customers. It's like a steak house enforcing a no refill policy on a guest that just dropped $40 on a steak. It's stupid from a customer relations perspective to enforce a policy like this. You are police-ing your customers who are paying a premium.
If that's not enough..
2. I'm not sure the actual percentage but I would say at least 50% (this is purely a guess on my part) of guests staying are on a Dining Plan and get the refill mugs for free.
3. Annual Passholders and "regular visitors" returning with old mugs... If you return to WDW on a regular basis I think you deserve free refills in your old mug. Again.. Disney is police-ing the regular visitors.. the best customers.
As a disclaimer.. I am saying all of this as a Disney fan... I don't drink soft drinks or the "Gold Peak Tea" that's got all the high fructose corn syrup. I'm the one buying the bottled "Honest Tea" for over $3 a bottle or getting coffee now that they serve the wonderful Joffrey's coffee (best improvement I've seen in 10 years!). So this decision does not directly affect me... but it's the point of Disney World trying to squeeze every single penny out of people that is really getting old.