heh. I think I know why.
That sort of idea would not show up in land search.
But honestly, I just don't think the community could handle something like that. Everyone has this mindset that they have to have borders. A common area, as you described, is pretty alien.
On Sentience, I made 1/3 of the island a common area. No one uses it (except me), though anyone can. And I think this is mainly because people like not having all the lag. I considered another island, but I'm not really comfortable with how Linden Lab is doing some stuff...

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