[Cainehill]

Also, seems that if assassinations are considered okay and legit, the war-spy stratagem certainly should be. I know I'd consider it just as much a declaration of war, if not more so, for someone to sit there assassinating my researchers, battle mages, prophet, etc, as for them to wait turn after turn for my PD to detect them.

In addition, there's a relatively simply defense : Leave PD at 9 (can't remember if PD has to be 10 or 11 to detect spies), and their stealth weapon won't ever be detected, thus can't take the province. Toss some regular troops & commanders in with the PD, and the province is just as strongly protected as if the PD was higher.