The page contains content that already exists elsewhere.
The page could be made easier to read with a significant refactoring.
The page should be merged with another page.
Information kept in the page is no longer relevant (e.g an XP conference announcement that is 3 years old) to most members of the community.
On the other hand, those wanting the page to remain do not think the proponents for refactoring have a case.
If we have a stalemate and do the delete, wiki contributors could diminish.
If we have a stalemate and do nothing, wiki pages will keep growing.
If pages keep growing without ReFactoring, they become impossible to read. WikiIsNotUsenet.
Usually one side is happy for wiki to keep growing, while the other side wants the existing information to become more usable before new contributions get piled on top of it.
I think the problem of WalledGardens is actually self-solving. If the WikiGnomes are right (and I think they are), they won't grow.
[The problem is not so much that they exist, it's that they often contain content that will never be found because it's not integrated with Wiki. Usually RefactorByMerging or RenamePage is a better solution than outright deletion of WalledGardens. -- JonathanTang]