I'm with Eirikr on this. Boring realms are a breeding ground for spies. Spies are just nobles looking to earn favour outside their realm, as they fail to get it inside.
I don't necessarily like spying but I believe it to be a byproduct of a failing leadership. If you can't bind your nobles to you as leaders, you fail. It's as simple as that. And distrusting them will only increase the likelihood of them turning on you.
Of course some people thrive on spying or are driven to it by IC actions and don't need a boring and distrustful realm to turn spy, but those people are rare, and negligible, or identifiable. If you play BM in fear of those few odd apples, you're depriving the majority of tasty apples the fun they came to get out of this game.
In other words: the fault for spying lies with uninspired leadership that fails to keep it's nobles happy. And that's not just about information, it's about gold and power and, most importantly, fun too.