I have to agree, except when splitting the wood right out of the truck , or with a lot of 6-12" rounds. With a pile like this: Vertical is the only way to fly. I've lifted those heavy sob's enough by this point.
I agree if splitting while unloading the truck if the rounds aren't too heavy. But can not agree on lifting a 6-12" round off the ground and up onto a horizontal splitter. Stand that sucker up!
I would say 99% of the time I'm vertical. Small job, or small wood may go horizontal. Any splitting session of an hour or more is definitely sitting. I am on my feet almost always. Splitting from a milk create is a well deserved break on the knees.