They don't add extra charges for not being a member for snacks. You get free upgrades and get a special line for concessions. So worth it if you get concessions normally ever, not if you don't. I think they also have another tier up that gives you 3 movies/month but I have no idea what that costs.
$15/year is pretty reasonable for the perks for AMC. Most theater chains are trying to charge monthly for membership which is outrageous