I think it's useful that people can add information, but you do have to treat it with care, just like an unsourced profile, and verify it for yourself. I still think it's preferable to having to start finding all the information from scratch myself. I've often submitted additional information for people in my family tree, and I've never had a memorial manager ask for my sources - which they might not always be able to check anyhow.
If you really want to control the information on a memorial, you'd need to become the memorial manager. It is possible to ask for the memorial to be transferred to your care; I haven't tried doing that, so I can't say how open people are to it in practice.