Dontrelle hasn't had an ERA below 5.00 since 2006, when he had a 3.87 ERA as a starter for the Florida Marlins. In the past five seasons, though, he has just been atrocious, and even got sent down to the minors in each year since 2008.
Despite all this, he was signed by the Phillies (pretty cheaply, at about $850,000 for 2012) to be a lefty-situational guy, also known as a LOOGY (lefty-one-out-guy)
His lacking performance would not have guaranteed him a roster spot in 2012 (he might have made it given a good Spring Training), but his friendship with Jimmy Rollins did.
Both hailing from Oakland and going to the same High School, it seems like their signings were a package deal: you can't have one on the roster without the other.