To me, they're mostly useless from a TM point of view. They're mostly (almost all?) concurrent with routes in other active systems. The minor benefits I can see are that they let the route be all drafted and ready to be flipped into usai when they become genuine interstates, and they let users add list entries for the usaif routes that instantly give them the new mileage in usai when the routes move there (via AltRouteNames).
Is there any support at this time for removing the usaif system from TM?