I haven't actually used Memory Maker yet, but from my research, I've gathered the following....
As the PP stated, the MM card can only be linked to 1 MB at a time. So, that person must be present to immediately add the pic to your MM account. As long as your all of your friends & family are linked up on your MDE, you can reassign the MM link to any MB you want, but still only one at a time. So, if you have 3 separate groups each going to a different place, only one group at a time would have access to the MM & the other 2 would not.
One good thing I've read is that you can still gather the old PhotoPass cards from the PhotoPass photographers stationed around the resort & then link them all up to the MM account after your trip. You can then download them all together & split them up amongst the different families. Although a bit more work, that might still make it worth the value for you.
I see the problem spots as being attraction photos & character meal photos. Digital copies of those are supposed to be included in the MM. I could be mistaken, but I don't think they give you an option on these to take a PhotoPass card to link up later??? So, in those cases the person who's MB is currently linked would need to be present in order to get the picture.
Hopefully, this is something they change in the future. It's seems completely counterintuitive to the technology of MDE & MBs. But, I imagine that this upgrade would have a lot of potential for bugs & glitches that would need to be worked out. Guests seem to be having enough trouble already just linking (and keeping linked) all their friends & family for the more basic funtions of MDE & MBs. I'm thinking Disney would want to iron out all of those glitches before adding another layer.