Just a quick quote from John Buckley's British Armour in the Normandy Campaign about tank riders:
"Some discussion also took place regarding the use of tankriders, whereby infantry would be carried forward into battle on the tanks themselves, debussing directly into action, rather than prior to it. The increased vulnerability of such infantry to artillery and small arms fire, and other operating difficulties, fused with negative reports from North Africa and resulted in the War Office dismissing such tactics, other than in exceptional circumstances. Nevertheless, in order to expedite the initial push on Caen on 6 June, and during the fighting in the bocage in Normandy, tank riders were pressed into service, despite the supposed disadvantages"