The Centurion had a 20mm Polsen cannon as a co-axial in the earliest versions and it was promptly dropped.
The realisation was, as with the pre-dreadnought, that an interim secondary armament is a distraction - as a tank you are there to kill other tanks, offer direct fire artillery support (HE rounds from main gun) and suppressive machine-gun fire.
I would expect the opportunity to take the time to realise that you have an IFV or lighter armoured vehicle in your sights, swap over to your secondary armament and engage is a luxury you don't have - either it is a threat, kill it fast (the priority is time to an accurate first round for a kill/your own survival), or it is not a threat, move to next target.
I'm sure I read about the use of training rounds in Iraq to achieve less lethal over-kill, subcalibre training rounds for "sniping" with something more like a .50 cal down the main barrel or training (inert) HESH to knock down walls without bang. In other words, if you have the safety of knowing you don't need to carry ammunition to take on peer threats, you can take your time and make do.