A couple or so points. If second hand, with a plug instead of a valve, I would be hydraulically testing the receiver for integrity – it may have had water sitting in it for some time!
Valves left very slightly open – as in leaking – may well wear like leaky water taps by cutting the seat with the high velocity wet air passing. Agreed not as bad as water cutting brass tap seats, but may occur.
I believe automatic steam traps operated with a bimetallic strip, which opened the valve when the water collected and cooled the valve (water is a poor conductor and any cooling caused the condensate at the lowest point to cool more as colder, insulated, denser condensate then cooled even more quickly thus opening the valve; steam escape almost immediately then heated the valve and it returned to the closed state.
Compressed air is an expensive commodity in terms of cost per unit work done. Leaking it away, however slowly, will mean extra power expense. Even if it is financially acceptable to the user, it is wasting our planet's resources!