The overreaction here over one crap summer league performance is typical for these forums and not at all surprising.
Yes, Yabu is playing for his career and he's been pretty much a bust to date - but I'd also hold off on final judgements till we get into the season and he's given a chance to earn minutes.
For the first time in his career he's being given that chance.
I would point out that we don't have much at all at the 4-5 so it's pretty much an open audition for playing time after Kanter. A Yabu who plays the way Ainge envisioned when he was drafted would help the situation, as right now we're on track to get routinely destroyed in the paint by Milwaukee and the Sixers.