Stats for the season so far:
Bogut 13.4 ppg, 8.9 rpg, 2.4 apg and 1.75 bpg
Sheed 12.8 ppg, 7.1 rpg, 2.0 apg and 1.58 bpg
Big Z 13.5 ppg, 9.5 ppg, 1.6 apg and 1.65 bpg
So you could definitely say that Bogut and Big Z had a fair claim to be All Stars as they both have better stats than Sheed. Obviously playing in a good team helps but I also think the NBA is conscious of handing out "token" All Star jerseys (especially given that it was an injury replacement and the East is low on centres). Sheed has been an All Star before and earned it while Bogut and Big Z probably have to do a bit more before they get rewarded.