Excellent question. At the moment, I use an older version of Reason as a ReWire client with various hosts. Perhaps using it as a VST plug-in offers benefits, but that's not a priority for me.
Geoff - as to Pro Tools, I found a neat trick. Peavey's ReValver amp sim allows for inserting VST or AU plug-ins, and there's an AAX version of ReValver. Many users have reported using ReValver in Pro Tools to run VST and AU plug-ins. I don't have Pro Tools installed currently or I'd try it to confirm, but it's an interesting hack. You get an amp sim as part of the deal, too