There's a difference between poor kicking skills and no kicking skills. There's a difference between being a poor judge of the ball's flight and consistently misjudging marks.
Jamar certainly reached his best late in his career, but he was always strong above his head and a decent judge when the ball was in the air (from memory, his main problem was not having the tank to get to the contests on the bigger AFL grounds and take the pack marks that become his trademark at his peak).
If Spencer makes it as some on here are suggesting (and by makes it, I mean nothing more that he plays 40 or 50 games) I would consider it one of the greatest elite-level sporting improvements in the history of sport.
I disagree that ruckmen in modern footy can be valuable while possessing poor basic skills. They are bigger liabilities than ever before.