I think exactly the same. Why Reason only works as Rewire slave?. Because it has no MIDI out.
If Reason implements MIDI out it will become the main DAW and Rewire host.
I don't know why external synths like Roland V-Synth GT can not be controlled by Reason sequencer and then recorded like audio track in Reason.
Reason force people to buy always another DAW that remains as Rewire host just for Reason's MIDI out exclusion.
Please Propellerhead: If you want Reason becomes the main DAW, just include MIDI out and CV (Note, Pitch and Gate signals) to MIDI converter rack module and Reason will be considered the main DAW for most people.