For cases where the user hasn’t opted out, the observation ID—the one displayed at the top of the page and in search results—also has the added benefit of moving the ball forward when a finer ID is applied.*
So, for:
ID1: Bouteloua
ID2: Bouteloua curtipendula
The community ID is still only Bouteloua, but the observation ID is Bouteloua curtipendula.
Prioritizing the community ID throughout the site would make it harder to find observations like this of B. curtipendula since they would be lost in the other Bouteloua spp. obs. That said, see also this affiliated request: Search by Observation ID or Community ID.
* ignoring the exception of taxa finer than species rank, which has been discussed elsewhere