More existing SBD means each of them backed by less STEEM. Even if the SBD isn't being paid out they still affect market price. Reducing SBD supply is why @sbdpotato is a thing in the first place and this is counterproductive to that, now along with countering SBD printing from inflation @sbdpotato also needs to counter SBD printing from @threespeak videos.
If not set to @sbdpotato, the video rewards beneficiary is better off sent to @null in my opinion.

Proposal to directly burn SBD from @steem.dao is great way to reduce SBD supply but if it's funded it will interfere with other funded proposal (if my understanding on how the DAO work is correct). Getting it funded in the first place is also very unlikely.