What makes me sure Watkins isn't ready for the NBA? Well, a good sign is that Watkins currently in the NBA. Even 17% Tolliver got picked up after the Spurs waived him. When Watkins was waived, he was relegated to the Chinese league -- possibly the worst professional basketball league in existence.
Watkins has good physical ability but his basketball skills appeared to be lacking. Having a shotblocker on defense would be nice but that player has to be able to survive on the court on the other end.
And I disagree that he was "VERY" close to making it. He wasn't much closer than any of the other training camp invitees.
I don't know what Watkins has been up to in China but if he has shown any signs of improvement, I wouldn't be against giving him a ten-day look. But to say that waiving him was some sort of travesty and unforgivable error by the front office is taking it a few steps too far.