SBD was this thing designed to make a price stable payment thingy. So that people could price things on a ebay like site for SBD, as they would for US$s.
But... it never panned out. And no one expected SBDs to shoot to $15 each. Nor for STEEM to go above $1. They expected STEEM to always be much less than $1, and thus an SBD would always be traded one way. So, the conversion mechanism is broken, and is still broken. And needs heavy thoughts on fixing.
Or, we may just want to hardfork all the SBDs into STEEM and forget them.
Someone could probably do a better job with steem-tokens.