I agree, any data recorded (or entered later in the song) should make it run longer.
I work with audio more than MIDI and my default track type in RB is audio. As a result I can just click a blank track and hit record for 2 measures and the song is now longer (everything from FX to synths now has time to ring out and empty the buffer) ..

If you want to use a MIDI track instead, hit Record in RB and record a couple bars of nothing, or enter a MIDI Event (as mentioned by jford) later in the song.
If RB sees *something* happen later, it will continue to play until then. Otherwise it acts like it reached the end of the file and stops.


Make your sound your own!
.. I do not work here, but the benefits are still awesome