If the controller says standby it's because he/she is doing something else and cannot give RTF calls suitable attenetion. Consequently, any reply is more on the RTF that the controller has to listen to - and he/she will have to listen just to make sure that the station calling is saying Roger and nothing more or that it isn't another aircraft calling.
OK, as a controller you get pretty good at half listening when all you're expecting is Roger, and your sub-concious picks up when you need to concentrate on the RTF - but any call in reply to an instruction to standby takes up some of the controller's thinking time when it is unnecessary.
Interestingly the UK's RTF manual doesn't give clear guidance on this but implies that no reply is required - I think that the ICAO books are the same in this respect.