The more I learn about why SBD was added into the whitepaper, it was to mitigate the effects of hyperinflation of Steem at that time. Today, Hyperinflation is no longer an issue.
As far as making it easier for merchants to price things and do accounting, there are POS apps that do that anyway. A merchant can price things as they usually do and the app woud convert Steem at the price it is into their fiat currency. That's even simpler than having to price things in Steem or SBD.