[Tagging] Is linear man_made=pier supposed to be assumed as implying highway=footway, unless tagged otherwise?

[Mateusz Konieczny]

During my recent mapping of marina-related areas I tagged 
piers walkable by pedestrians as both highway=footway and man_made=pier.

For an example see
http://www.openstreetmap.org/?mlat=54.03130&mlon=21.77232#map=18/54.03130/21.77232

This practice is currently not mentioned at
http://wiki.openstreetmap.org/wiki/Tag:man_made%3Dpier

From taginfo data it seems that some tag this way, but it is not
popular - 2.28% of man_made=pier have also highway=footway or
highway=path

Further 0.44% of man_made=pier have an another highway tag (findable
with http://overpass-turbo.eu/s/sxQ - most of these are
highway=service/pedestrian/steps).

https://taginfo.openstreetmap.org/tags/man_made=pier#combinations

Would be a good idea to encourage a more explicit tagging?

Or maybe every single man_made=pier is walkable and highway=footway
should be considered as implied, unless other highway value is tagged?

And routers/renderers/others can consider man_made=pier and
man_made=pier with highway=footway as the same?

Or maybe it is preferable to be explicit with mapping?

(I am asking here both as a data consumer and mapper as my experience
with sailing-related things is limited)