.....less necessary because they can see the squawk change from 0234 or whatever to 7000 so they know you have moved on
True to a certain extent, but if you do this (in fact whenever you change squawks), PLEASE switch your transponder to standby while you change the squawk. Situation the other day in which a pilot did this after leaving another unit's freq without saying bye bye for whatever reason. Unit squawk changes from XXXX to 7700 and stays on the emerg code for a good 5 mins til the acft called me. Better than a shot of caffeine to get the heart rate up!
Even if the right code is eventually entered it is annoying to see various numbers/callsigns pop up as the pilot trawls through the numbers.
Of course this can happen even if the intention to QSY had been acknowledged.