Why is to much venting no good on a radiator?

Steam will take the least risistant path ...We use resistance to direct /control the steam .We use the resistance to balance the system ... We want to fill the mains with steam first ,next the riser- radiators.. We normaly want to fill ,all of the radiators ,at the same time .. Small to large, same time ....

too fast can also jet the steam right to the vent,
and not let the rad heat to it's potential.
a delicate balance all around.