I think it is probable that HIVE is a gas (sort of, if RC persists) and governance token. In fact, that's 99% of what it is now.

The main use case for HBD is most everything else people want to do on the chain in terms of base level transactions and fees as well as being a sink for capital people want to stash for yield. Currently account creation fees are denominated in HIVE, for example, but that's kind of dumb since that jumps all over the place for no good reason. When SMTs were a thing, the fee to create was originally going to be SBD for similar reasons. The promoted page worked that way, way back in the early days. We're using it for DHF, where longer-term project funding works well with the stability and contractors using and holding the token themselves is fine since most expenses are fiat-denominated or regardless of denomination are usually tied to purchasing power in some manner.