The output has to be burned  otherwise there is no cost of expressing
any number of alternate opinions the same time. 

Tamas Blummer
Bits of Proof

On Dec 15, 2014, at 3:55 PM, Isidor Zeuner <cryptocurrencies@quidecco.de> wrote:
For every participant who could try to decide about the adequateness
of the cost, no direct effect comes from the difference between Proof
of Burn, and approaches which keep the Bitcoins inside the set of
outputs that can still participate. So, any notion which
differentiates with respect to this must make some assumption about
what defines the interest of the Bitcoin nodes as a community.

Best regards,

Isidor