we do. being a machinist in the US means being able to convert between decimal inches, mm, fractional inches, and things like feet all the time. not to mention other completely arbitrary units like sheet gauge, fastener number and inverse thread spacing. there are also decimal feet for carpenters.

this for me is the real appeal of metric, not that somehow a meter is magical, but at least there is one system, with a consistent set of rules, that allows us to do some magic things like tell the approximate volume of water given a weight.

edit: omg I forgot about nominal wood sizes. the underlying system actually has different units based on the material and the usage. copper gauge is not the same as steel gauge. thats pretty hopeless. for precious metals we also have the pennyweight