I think the answer on F&WF use for vouchers is no...although since they have not offered the plan during F&WF time I suppose there is a chance for snack options....I guess whether it is worth it or not depends on your dining styles/preferences....for us it is definately worth it. We like to try new sit down places as well as the old favorites and it is nice to be able to order appetizers, entres and deserts without considering cost. In the past we have tried other meal plans that included 2 sit down meals/day and we nearly killed ourselves!! By the 3rd day we felt too stuffed to move and nothing even looked good to us....this option is much more flexible....for us 1 table service meal/day is plenty, the CS meal will keep us from having to take so much time out of the day for sit down meals and allow for a lighter meal....also this plan allows you to share meals if you wish and then use the stored credits for the signature restaurants....which to us is another helpful feature that will help us not to eat ourselves to death (we have so little self-control!!)
However if you never want to eat at counter service places or you want to spend a lot of time wandering the F&WF then the dining plan probably isn't the best choice for you.
Best advice would be to just play with the numbers...look at places you would normally dine and what you would normally order and see how much you would usually spend for a week....and then compare to the $490 the dining plan will cost plus whatever else you would spend on food that would not be covered (F&WF, etc.) and see how it looks.
For us it was an excellent value, not to mention the added fun factor...but your situation may vary.