There's no chance that Parker is more mobile and can pass better than Harrison. That's why Harrison is there. He has the ball skills of a 5/8 and is a gun player. Parker is a prop in the number 13 jersey for the Broncs. He plays tight, and the only passing he does is an offload.
No one is knocking Harrison. He rarely misses a tackle in Origin. He rarely knocks-on in Origin. He rarely drops the ball in Origin. He backs-up well in Origin.
He is smart enough to be a team player in Origin. That is an intelligent decision, in and of itself at Origin level.
It is not his role to pass the ball. He receives the ball and unhesitatingly attacks the opposition defensive line. He defensively covers for Thurston. He gets little of the 'glory'. He is never the 'star' of the team. He just quietly goes about his business, with no fuss and few mistakes.
He doesn't give away penalties. That in itself is a huge plus.
Parker will do all of those things, just as well at Origin level, plus he has the ability to put on a sidestep and get away the nifty offload in a tackle. Parker forms a nice combination with Petero, Hodgo, Slammin' Sam and Benny Hannant.
Food for thought.
Not knocking Ashley. I've acknowledged his pluses and he basically doesn't do anything much wrong.