On siege a sumpter is very useful. I spawn with one every round. It multiplies my effectiveness.
On offense I can speed to the objective. I may die, but on my next life I'm back fighting the same guy in like 12 seconds. It's awesome.
On defense, I can jump down from very high walls to objectives or chokepoints. The horse usually dies, but I am unharmed and can fight on. Saves a ton of time having to locate and run down stairs.
So its kind of a disposable transport system. It's not to be used for fighting on (tho sometimes I do get a lucky drive-by swiping kill).
TL;DR: I love my sumpter. Her name is Butterstuff.