More use of the blockchain benefits STEEM holders directly. In order to engage in a large volume of transactions (for example as a merchant or payment processor, but perhaps even someday as a regular user) you need to hold sufficient SP. That defines your bandwidth allowance, is the substitute for transaction fees in this system, and in turn contributes to buy-and-hold demand for STEEM. The need for more SP (i.e. the implied fee rate) will increase as usage increases and blocks start to fill up.
Despite the currently-low usage (most blocks are relatively empty), I experienced this directly in needing to power up more SP to my market maker bot to avoid having its transactions blocked by the bandwidth limiting (though it was still only a small amount).
However, that benefit (implied transaction fees) being said, I personally believe the greatest benefit to STEEM holders remains helping to build a larger user base for the entire platform and a larger user community, for the reasons described by @ats-david. Without a large user base this entire platform will inevitably become (nearly) worthless. We need to keep our eye on that ball. I guess it is a matter of opinion but I personally think that the presence of SBD helps a lot more with the critical issue of growing the user base than would the incremental simplification of removing it. The systemic risks and other issues are also a factor though.