I think the main thing with working in RB you have multiriffs, so no matter how far you are into a song you can still change pieces you don't like rather than having to go back to RB, load a mixdown from the daw into RB to generate up the bits you want to change or new pieces you want to add, like if you decide it will sound good to mute the other instruments and have a bit of piano at the end of that second chorus.
Unlike REAPER, Sonar will only operate as a ReWire Master, so you can't have a MultiRiff playing in RB to a section of Sonar's track.
With the ReWire VST in RB you can do this with REAPER or other DAWs that will ReWire either way.

http://www.energy-xt.com/index.php?id=0115
the free version of the ReWireVST works enough to use with RB,
if you load it into a midi track it will show you in it's drop down what apps you have that will slave to RB, so you will have a solid stable DAW to load as many VSTs as you like but have multiriffs to add to any section of the DAW while it's playing in sync to RB that has no load on it to cause any crashes.

I suggested a way to do MultiRiffs in Biab when it gets ReWire implemented http://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=378682