Back in the Exile 3 days, I did h** e*** once to be able to summon a doom gaurd. Sadly I think I accidentally hit one and they just kept on popping up. Soon, I not only was surrounded by a legion of doom gaurds, but I couldn't tell which was hostile and which wasn't. So, I'd assume not allowing people to capture doom guards was for good reason.
What did happen after that unfortunate incident? My computer took forever to do anything and I ended up restarting my computer.