ya you will melee with left click by default. but there is a seperate melee button near the bottom of the list in the bindings which will use your claws and leave your claws out if you have plasma selected (so selecting your claws at the same time as performing a melee hit). if you have another melee weapon it will just attack with that weapon.
so if you have plasma and you just left click you will melee but plasma stays selected.
if you use the melee bind in the options with plasma selected it will melee but leave claws selected after.
thats why the cancel works because you can cancel the melee animation by selecting plasma again as soon as the hit lands and it cuts the melee animation short and selects your plasma.
for this reason i would not actually rebind that to left click as then everytime you melee with plasma selected it will select your wristblades, play with it and you will see what i mean. so use E or any other key as an alternate melee for this cancel trick and leave normal left click alone. I am going to edit the thread with this.