Very interesting question, but it is not as easy as allowing free two way conversion. SBD's are essentially loans, promises to pay steem to the holder in the future.

Currently only the steem-blockchain can issue SBD, backed by the ability to print more steem in the future.

When people should issue SBD, allowing the opposite way transition, these SBD need to be backed against fluctuating steem prices. How this can be done is well known on bitshares smartcoins. But it is much more complex than adding a butting to turn steem into sbd.

So it can be done, but it involves bigger changes and will make the two-currency system even less understandable for the average user. Therefore I would oppose such a change.

The advantage is that the concept of SBD becomes easier to grasp, but that does not help adoption as everyone is happy when sbd is worth "too much"