elaboration as requested:
You're either ready for departure or you're not. You may believe you will be ready on reaching the holding point but there is no guarantee, and most of us with experience will be able to recall occasions when situations have arisen during taxi which affected our ability to depart.
Why not wait until you are ready before clogging the freq with unnecessary (and in my experience, often misunderstood by non-UK ATS) RT?
It seems the intention of crews who call "ready on reaching" is to influence ATC into expediting their line-up & departure. Whether made with innocent intentions or as a result of crews rushing to get away, I think at best it is unprofessional use of non-standard RT, and at worst ambiguous and potentially risky.
The standard CAP413 phrase is 'ready for departure'; if, and only if, you are.
Thread creep, sorry. I've no more to elaborate on!