The problem with a subscription board is that it requires a subscription.
Many times a post on the present "locale" state message boards is a quick way to announce important state activism, issues, and meetings.
Let's say someone posted a notice, for instance "Recall Joe Blow", on the new state board.
How many of that state's residents actually have subscribed?
I'd say no more than 10%.
Then how will all [state] residents know about the recall message?
PING everyone flying the state flag to the thread?
A lot of people don't check spam pings anyway.
Another issue is "at-a-glance" perusal, but I'll let someone else elaborate on that one.
Flying a flag on your profile page would have to automatically subscribe you to the topic. Converting to this system, everybody that has a flag would automatically be subscribed to the state topic.
This would also allow people who call several states their home the ability to keep current with each. (That is, by subscribing to multiple state topics.)