Bummer had a 6.79 ERA the year before he was traded. They were never getting anything good for him. A potentially smarter move would've been to hang onto him for another year and see if he'd return to form and match his FIP, but that's not the argument I see being made. It's just bitching about getting relatively fair value for a guy who put a ton of people on base and had only one real standout year as a reliever.
He's been good since leaving the Sox, but still has the same issues we saw here. a 3.5-3.6 ERA reliever is a decent piece but not someone you expect much in return for in a trade.