You have to do a little pencil pushing and decide for yourself based on what you'd use the wishes for. The silver add-on costs you an extra $68 per night, which means each wish is worth $34. As long as you'll be spending at least $34 for whatever you're using the wish for you'll come out even or save money. I don't know who all is going on your trip, but keep in mind that kids' wishes are cheaper, but can be used by adults, so that would actually make all the wishes worth more.
For us, the silver plan was a savings. We went on the Keys to the Kingdom tour (regularly $58 a person), the Hidden Treasures of the World Showcase tour (regularly $58 a person), went to the Hoop Dee Doo Revue (about $50 a person), the Spirit of Aloha Luau (about $50 a person) and saw the Cirque du Soleil (about $80 a person, and cost two wishes). For us this meant a savings of around $90 per person. Usually things like tours, shows and spa treatments are worth the money, but things like meals aren't - unless you can eat $34 worth of food.
Check out mosuesavers.com for some ways to make the most out of your wishes and look at allearsnet.com for info on prices of this so you can add up what you want to do and see how the price compares between getting the plan and buying things individually.
Also, I thought I'd let you know that we split our six-night trip into two reservations for three nights each. That way we only paid for the add-on for three days and still got to do everything we wanted.