I have never used Hobbs, as in the vast majority of installations, all that's telling you is time with the battery master switch on.
That can be the case but the standard is that the Hobbs is activated by oil pressure so not only the master switch. In the case of the Rotax engines, commonly installed in microlights, the manufacturer stipulates start up to shut down and this is logged in the engine logbook. Therefore the Hobbs/Datcon meters activated by oil pressure do actually reflect this requirement. It isn't a matter of being a microlight. This is the case whatever type/class the engine is installed. The Continental 0-200 is commonly installed in microlights and airborne times are logged in accordance with the manufacturers approval. Whatever class of aeroplane the engine is installed in it is the manufacturers design approval that applies. I have never known of an engine that is stipulated to be maintained on tacho albeit this is the simplest and unambiguous method.