If they are near my territory, they tend to last a while. Don't want to risk popping specters early game. Plus, the chance of getting a late game tech is rather nice.
Even if they are rather uncommon, I don't think that's reason to not implement the idea. The world is already full of incredibly uncommon things. I have never, ever seen any of the giant monkeys. That circle of Geilin or whatever its name is has never appeared for me. I got Mary Morbus once, a long time ago. Other people get them more often, I'm sure, but this has just been my experience. Even though I don't benefit from them, it's still nice to know that something is out there, even as just a possibility. Adds nice depth to the game.