Fans have the right to boo, but it isn't right.
BUT, Renteria does deserve those boos, he's hitting a paltry .239 when we could have gone after Julio Lugo for a year for about 35 million dollars cheaper. Lugo probably would have given us the same production. (.280 10-15 HR's)
Millar doesn't deserve the boos though, it's expected of him to have one crappy half, and one dominant half. He also doesn't make even close to the money as Renteria does, plus Millar is an excellent teammate.
Bellhorn is different, he deserves boos, but why bother? Not one of you can honestly say, that you expect Bellhorn to do anything when he's up. I sure as hell don't even watch his atbat's, because I know he's going to walk or strikeout. Have to give some props to him though, he is a good clutch hitter.