SBD aren’t supposed to be a speculative asset (something for investors to put money into). Steem and SP are the things people should be investing in. SBD is supposed to be a stable currency so that businesses who want to launch a platform that runs on Steem can predict their future cash flows.

If a business wants to sell goods or services for SBD while SBD is a speculative asset, their customers are incentivised to hold on to their SBD in the hopes they will increase in value, rather than spending them to purchase goods or services, which are not likely to increase in value.

I hope this explanation makes sense.