I am also on the fence.
At first I viewed it as unethical, they'll catch on and that will be the end.
However, that's not how Disney works. They are going to figure out the best way to keep the cash flow without too much disruption to their "fans" which is why they are so successful. People keep paying the higher prices by any means possible (i.e. Sharing something like MM bc it's possible).
I am sure that what they are charging and profiting from with memory maker plus individual photos sold not on MM is way more profitable than the old way of printing those 100's of pictures you see at the end of the day or ride that a majority of people didn't buy. Realistically, how many theme park pictures do you actually have hanging up in your house compared to the amount taken at the parks? (I have 4 in a storage box from 2001 - I quit buying pics from theme parks after that year.)
Also, Disney says CM's will take pics with your camera instead if you choose, but for some people that takes away from their trip. I think what we are paying for is convenience and the experience. If a group decides to share MM, there are drawbacks to that so that still leaves the others willing to pay full price to not deal with that.
If Disney feels people are cheating the system or they aren't making a profit, they'll change the system. They always do.