There's no reason not to keep Spencer on for another year, given the size of the rookie lists. However, much of the positivity about Spencer is really just about hope.
I think that's fine - a season or two back I posted a list of all the AFL rucks and their draft origins, and they're certainly not all high picks.
However, it is reasonable to conceder that apart from being tall and having a bit of aggression about him, he's not showing much at the moment.
Hudson's hardly a mobile ruck either.