I haven't played cav in a while but here's my opinion:
Great Lance is something you keep in your inventory but don't include in your default equipment. On some maps, you can take advantage of it. Other times, trying exclusively to couch enemies will just be a waste of time (or maybe not even possible). When the map is right, couching enemies is like shooting stabbing eels peasants in a barrel field.
If your horse dies, the light/normal/heavy lances serve as a mediocre polearm. No side swing, but hey you have more reach than a war spear. If you've got a shield and lance, there's still plenty of fight left in you once you lose the horse. With the great lance, you have to have a second weapon. That means no shield above 1 slot unless your secondary weapon is a sickle or dagger or something. There's probably a decent build with a great lance/1h/shield and no points in polearm wpf, but it is far from optimal.
The "problem" isn't that the great lance is bad. It is good at what it does, but it can't do anything else. Every other lance option is more versatile, and people would rather just heirloom their heavy lance and be done with it.
Light lance/double sided lance are surprisingly fast with high polearm wpf. Feels good to beat down a non horse-haver that killed your mount and ran up to you alone for an easy kill.