So I originally did like the tag and I think that it’s better to have a consistent list of values of highway=* but there is a catch.
Notice that there are way more highway=* values that depend on functionality rather than importance:
- living_street
- service
- pedestrian
- track
- bus_guideway
- escape
- raceway
- busway
Not to mention that such a switch of schemes is impossible because highways are the thing that’s supposed by almost every data consumer and a sudden re-tagging of all motorways in Europe as highway=trunk.
Additionally, the majority of Europe uses highway=trunk to mean roads of higher quality, often access-controlled, meaning not depending on importance which is what I recommend fixing up first.
Instead, I think we should move away from the highway=* importance dependence proposal and move these values over to the importance=* tag.
Overall, this situation is being discussed further in this thread with my latest stance on it in my latest post.