I think the main usefulness of a mtb:scale=passable or mtb:scale=impassable tag is of a temporary nature: it highlights a trail for survey by a mountain biker with possibly some OSM experience. Otherwise it only has a “better than nothing” value: it suggests that a trail may be easy or very difficult, but the end result should be a trail tagged with a proper mtb:scale=0-6 tag.
An mtb:scale=passable or mtb:scale=impassable tag should be mostly for armchair mapping and for use by mappers without mtb experience.
An armchair mapper may have read a blog about an mtb rider describing a trail as “fun” and with beautiful views, but without sufficient information about its difficulty. Or he may have noticed that a trail has a lot of heat for mountain bikers on Strava. This mapper could then tag the trail with mtb:scale=passable, in the hope that an mtb-experienced mapper will be tempted to go and try the trail himself and then tag it with a proper mtb:scale=0-6 tag.
I recently add this path Way: 1383928851 | OpenStreetMap because it showed as being used by mountain bikers on Strava and I didn’t know it yet. I added sac_scale=mountain_hiking and trail_visibility=good because as an experienced hiker, I feel capable to add them. It would be nice to add mtb:scale=passable too, to show that it is being used by mtb and as an invitation to survey it for mtb:scale=0-6 As suggested above, it could show up for Strava users as a request for review with 7 options with brief descriptions to choose from. I would also like to be able to add mtb:scale=impassable to this way Way: х. Амбарица - з. Ботев (844312515) | OpenStreetMap : it’s part of a long distance hiking route that is also popular with mountain bikers, but I think this part should be avoided by them (that it has sac_scale=demanding_alpine_hiking already gives a clue). While we were there we met a dirt biker who got stuck there because it’s impassable and too narrow to turn around…