The idea is this: when a plug-in crashes, the host shouldn’t crash with it. Now, the developers of Reason are obviously very proud of the stability of Reason, but that isn’t the issue here. However stable or instable your host is, the notion is that a plug-in shouldn’t be the reason that host crashes.
It seems like a GREAT feature. Sandboxing is the answer and it’s something that I’ve asked to Steinberg for ages.