I picked #4. Spying was part of pretty much every war in history, so it's only normal it happens in BM as well. Personally, I've been a spy once. My character was asked to stick his neck out and travel to the enemy realm as a plant. He was banned on some fabricated charges, hurled a couple insults at the ruler for good measure, and fled to the target realm to hopefully ascend the ladder and gain access to their higher councils. After about two weeks I got so bored in the realm I left again.
Thrice I've recruited a spy to report on an enemy realm. The first time, I just sent a letter after a big battle asking if anyone was tired of being sent to another defeat and wanted to help facilitate his realm's final defeat and surrender, thus ending the war. I got 3 or 4 positive replies within the day. The second time I took a noble who had a bit of a falling out with our ruler and had him 'defect' to the enemy as a plant. The third time I contacted an old enemy (who boycotted a tournament I hosted) who had led a failed rebellion in his realm, after which he fled to his ally (my enemy) and agreed to spy for me, and in return he would 'get what was coming to him'. After his usefulness expired he was kicked to the curb, so to say. I've heard of others outright buying spies, too. All that I consider acceptable forms of subterfuge.
But then there's people who just recruit their RL friends, more often than not people who've never played BM at all, and have them join enemy realms with the sole purpose of spying. They have no IC reason at all for doing so, their characters just up and decide that they want to be spies, and they know exactly who to talk to in the other realm. That's incredibly cheap in my book. It's not even that difficult to plant or recruit a spy through 'acceptable' means, but they won't even take that trouble, probably because recruiting RL friends is a surefire way of making sure the spies don't get a change of heart and stop reporting, or become double agents. For shame...