1. Health - Many offer health guarantees, which can range from the first few weeks of settling in, to those which include guaranteed passes on required health tests. How many rescues offer health guarantees?
2. Temperament - Responsible breeders breed to a standard, and that standard provides a long heritage of expected temperament. When buying a purebred, you have a base guideline for the breed, which allows you to better select a breed for your lifestyle. When you buy a mutt from a rescue, how much do you really know about its heritage?
3. Socialization - Since breeders tend to breed for the show ring, or even simply for pet homes, they are inclined to socialize their dogs with people and with other dogs. Also, back to that standard... They also know if their chosen breed is one that is gregarious and easy going(like a Lab) or more reserved and less inclined to respond to proper socialization(like a LGD).
4. The Downside of the breed - A responsible breeder will turn away buyers who do not fit their breed, or who make them uncomfortable. A responsible breeder will make a potential buyer aware of the issues they may have with the breed at hand, and may suggest an alternative. They will also warn against those less scrupulous breeders, such as those breeding without the health of the animal in mind.
5. A Lifetime Home - A breeder will take back any animal they have sold, should the owner no longer want them. This provides a safety net, in case of job loss, sudden illness, accident or death. The responsible breeder will then make every effort to find the dog a new home, on their own dime and on their own time.
6. Housebreaking - Many times a breeder will have started work on paper training, or even fully house breaking a dog. A good breeder will not only allow, but encourage you to contact them with any training issues, so they can advise.
7. Known Heritage - As breeders tend to work on lines for an extended period of time, you have the benefit of not only knowing about your pet's parents, but also grand parents, great grands, and sometimes even further back. This can help when it comes to diagnosing illness, behavioural problems, or simply help you pick the right dog for your family.
8. A Support Network - Ethical breeders love their dogs, and love their puppies. They want their puppies to fit their new homes and succeed in their lives. They will encourage you to contact them with updates, issues and triumphs - many have a "brag" page on their websites to post the achievements of their extended canine "family."
I know I've forgot some. I could go on for longer, but I've already refuted every point from the original post, plus added one of my own. If you have a reason I've forgotten, please post it in the comments!