I actually think that Spearmon did about as well as he could naturally in the 2, and his 100 was obviously as fast as he could go.
As far as a waste of talent comment from johnson, yes Spearmon could have done and could still do something incredible in the 400, although the 400 is a very hard and grueling event.
Spearmon is probably one of the few people that can go sub 10, sub 20, and sub 44 at the best of their ability, and he has 2 of those and the third is just a lot of pain and hard training which could end up ending what he has left of his time on the track.
Not everyone can train as hard as MJ. Not everyone would want too.