Being a tonsured reader, I can tell you that during the ceremony for tonsuring is the only time I've seen it used. Since becoming a reader is the first step into the minor orders of the clergy, it's only fitting that the candidate wears a clerical vestment to symbolize it. From the time you become even a reader, you are to give yourself up in service to the church. Therefore you wear the short phelonion, which is symbolic of the robe that Christ wore during His passion.
Then why the old ritualists servers(taper-bearers) wear it even after the cherothesia,and greeks never use it in any case?