you will be surprised at how often people forget to switch round. It's basically giving the pziv a free shot if the sherman get caught with HE.
Switching round type also take a bit longer than simply reloading, so that's another minus.
As i know,the M4A3 sherman can kill infantry using the common shell, its AOE radius is 2.0(if I m wrong correct me).If you want to kill infantry insanely ,switch to the HE shell,if you want to fight both infantry and vehicles, use the common shell (named AP shell ingame).