I may point out that even the bay12 servers always had greifers, even after reasonableman started to only hand them out on PM towards trusted people
unless there is a way to confirm who caused the grief, moderation is useless
land-claim flags are absolutely necessary, else it will be as duke said, houses will just be markers for loot
the game would only be about who can hide is place the best, everybody working in isolation. fuck that
If someone's tearing down structures, and there are at least a few people on the server, they are going to see it. I can't see anyone from the community here greifing on our community server, so it's not like it'd be hard to tell who did it.
I can't give any statistics here, but I'd be willing to bet that most of those greifers are people who haven't payed for the game and don't give two shits about and just want to have fun pissing you off.
EDIT:
There is one worry I have for land claiming. What happens if I'm in a claimed bit of land and I get stuck in a 2 tile deep pit?
There are worse possibilities, say someone goes and claims all the land not already claimed. Or say someone claims the land you were building on, but didn't yet have a land-claim flag of your own yet.
Land-claim flags could be used FOR greifing. It wouldn't be EASY, but it's more then possible.
EDIT2: I guess i can extrapolate SOME statistics.
A Poll on the forums, of which 543 people responded to, has 53 people claiming they will greif. Which is 10%. That's pretty a pretty bleak number, however, this is an anonymous poll. Another poll with 104 respondents, 8 of them said they would be terrorists, Making an obvious connection, that puts it at roughly 8%, still pretty bad. I tried to find a thread someone started where he essentially said; I plan on destroying and killing with no obvious personal gains, who else does? However, I can't seem to find it now, Personally I think that would better represent the situation, as that would require people to actually state that they plan on greifing.