SBD is used because it was designed for commerce. A stable peg to $1 makes it easier to have a consistent currency for products and services.
If you want to sell a $20 baseball cap, would you rather use a currency that is 10 STEEM one day, 8.5 STEEM the next, and 9 STEEM 5 minutes later?
Unfortunately, the system is currently broken and SBD is not properly pegged to $1, partially because external influences and because witnesses were not given the tools to properly peg SBD down, only to push it up.
I understand SBD more as a gateway between fiat and STEEM, from a costing point of view. With only a 5% SBD/STEEM debt ratio target, and with rewards being paid in both SBD and STEEM now to keep below that 5%, STEEM is obviously meant to be the medium of mainstream transactions. We can pay with BTC, LTC, ETH, and all those are just a volatile as (if not more than) STEEM. SBD is probably more volatile than STEEM right now too.
I'd say for commerce SBD is more of a cost reference where you can easily convert 1SBB into STEEM using Witness price feeds, and then make price changes in STEEM on the fly quite easily directly on the platform. So if 1USD is =1SBD, converting to STEEM from USD wouldn't need a 3rd party feed coming into the equation.
I absolutely agree that the broken peg is a problem, and hopefully can be addressed eventually to bring some stability to it. It has been a great run in terms of higher post rewards in $ value, but that is where the barrier to entry is for commerce, and growth in the long term. Has there been any further developments on that front, or are we still in "wait and see" mode?