Not sure where you're getting the idea that Bosh is a good defender. He's pretty soft on that end of the court. Howard is certainly a menacing presence, but Bosh's value is and always has been on the other end of the court. And Parsons is nothing special on defense either.
Beverley is a pesky defender for sure.
I do agree that Bosh is worth it for Houston for the professionalism/personality and for his scoring versatility, though.