Permissions in more than one town becomes an issue for a few reasons, such as betrayal and alternate towns.
While I do support being able to build in others' towns, this is probably not the best way to do it. An alternative would be the "level 0" zone I suggested as a town perk, which tries to eliminate the issues of multiple town permissions.
I think where this comes into play is when a person who is already a member of a town goes to help new players make their own town. It becomes a hassle to leave your town to help another town then have to walk back and get added to your town again.
Remember the reason we've not been fond of this in the past is because it creates even more small-population towns. Many towns are left to rot because players get bored with their towns and want to go play in somebody else's town for awhile. While this at a base level seems fine, it does leave many ruined towns around the world.
That said, I'm not saying we won't do it, just make sure you guys remember the why on this one.