It really depends on what kind of person you are I think. If you are someone who likes to get group photos during your vacations, MM is worth it for that alone. We haven't always purchased it for every trip, but usually do. For us, it makes it really convenient and easy to get the pictures we like - without having to fumble with our phones, hand our phone off to a CM and then of course it always locks before they take a picture and I have to unlock it and then dash back to pose. You get the drift. When I've broken it down in the past, we get so many MM photos that they cost less than $1 per photo.
When we get MM, we have 2 sets of photos and between the two, they are a pretty comprehensive memory of our trip. We use our own phones to take pictures of things that capture our attention - animals on the safari or how yummy my Dole whip looks, for example - and then MM has all our ride photos (which usually make us laugh), character meets, magic shots (which are fun!), and group shots that include everyone.
I do make a photo book from every trip that incorporates ALL of those different photos, so it's important for me to have them.