Defending multiple entry points is extremely prone to failure. One of them failing is enough for the entire castle to be stormed very fast. Also, enemies reach entry points much faster than the flag, on most castles.
Defending the smallest number of chokepoints works much better as you can concentrate your forces.
It is true though, that the art of slowing down the attackers rather than blindly killing them seems lost. I often find myself swarmed by attackers when I stay too close to an early entry point, but when it happens I don't try to kill them, just slow them down and annoy them, which usually proves to be much more effective. The longer I stay alive, the longer the delay until most of the mob can advance. And with a shield you can stay alive veeery long.
Similarly, the art of pushing defenders at a chokepoint rather than killing them is not very widespread either. Most attackers will try to kill the first defender they see, even though just moving past him attracts the attention of the defenders and opens up the way for the attacker behind you.