I think that the wiki type editing is a complete non-starter. Most of those of us who take care to create good pages do not want them fussed with. That said, I agree that people besides the page maintainer should be able to contribute to pages. I think there are two good options that might strike a balance between owner control and user contribution.
1. Perhaps the page owner can identify which sections or subsections are open to public editing. If, at the outset of page creation they want to check a box that allows blanket editing privileges to all (a de facto wiki) then that is to their liking. If they only want certain sections to be public then there ought to be a box to check or a drop down that allows that section or sub section to be publicly edited. By doing this, then the owner has control over what is edited while those who want to contribute something have the ability to do so in the open areas.
An open section at the bottom of the page, as previously suggested would also be a good option for odds and ends, in addition to my aforementioned suggestion.
2. The other option, which could work in conjunction with my previous suggestion or separately, is to bypass the elves in adding editing and admin privileges to users. In this scenario, if a user wants to contribute, they need to apply to the page owner to get editing privileges. Perhaps a stable of contributors can be developed for really popular pages, all under the overall direction of the page owner. For those concerned, power point can be apportioned to added editors based on volume of owner approved content in addition to those awarded the original developer of the page.
3. Either way, the separate issue of inactive users and their pages needs to be addressed. However, that is a separate issue from the purpose of this thread and should not distract from the larger issue at hand.