I would say they're ABSOLUTELY worth it, but it probably does depend on how far away you are from the food court/refill stations! We've stayed at the WL 3 out of our last 4 times, and we've gotten refill mugs every time. (Well, ok, actually we've only bought mugs on two trips because, on the middle trip, we were still allowed "lifetime" refills. Then in 2003 we bought new mugs because the policy had changed to free refills for the duration of your stay.)
Anyway, we used those mugs a LOT and probably saved a great deal on drinks. We'd get coffee every morning...we'd take a mid-day break back at the hotel and get sodas...and then we'd get at least one refill at the end of the day (sometimes 2 or 3...soda and then hot chocolate). So we'd get 15-25 refills per person over a 5 day trip...Definitely saved us a lot of money!
On the other hand, when we stayed at POR (then Dixie Landings) back in 2000, we didn't get the mugs because the food area was quite a walk from our room.