Usually someone on FSB actually wants to fly. It seems odd to have a perfectly good FSB sitting there while a non-global flight is delayed 2+ hours waiting for another reserve to show up.

Since an FSB is not replaced once one is used, it’s not like using an FSB on a non-global trip would cause more FSB’s that day. If scheduling wants to use an available FSB on a non-global trip, go ahead and use them. I think most people would rather fly once on FSB rather than just sit there.