Proof of burn is riskier than PTS/AGS as well. Keep in mind:
"Disclaimer: The code being released here is of alpha-quality and under heavy development. You should expect to encounter significant bugs when using it. It is even possible that a bug might cause you might lose some money. The Counterparty team will do everything in its power to prevent this from happening, but this technology is very new, and the implementation is not yet well-tested. In particular, we may at some point have to change the Counterparty protocol in a not backwards-compatible way, if such a change is necessary to fix a bad bug or an exploit. As always, don't invest more than you can afford to lose."
PTS and AGS have no such issue. If the test bitshares fail due to bugs, they fail; but test bitshares 2 will simply take on the mantle and carry on.