For software to validate correctly, it is not opt-in. The continuation of the chain thereafter depends on people actually running the hard-fork code, not just being aware there is something happening. This situation applies to soft forks as well. After activation, it depends on people running it, most notably miners, otherwise the soft-fork is no longer enforced leading to a hard fork. Awareness alone does not ensure full validation capability is retained during a soft fork. Therefore, these differences seem insignificant enough to merit treating soft and hard forks equal in terms of the coordination features afforded through the versionbits. Witness the long preparation time ahead of SegWit deployment for wallet providers, miners etc. to coordinate to support it on their systems.