It's not what it seems all the time. As normal pet owners, it's definitely best that we adopt. However, getting a puppy from a legitimate breeder also has its advantages, especially for show type dogs, service dogs and police dogs as training is required to start while they are very young. This also eliminates issues regarding health histories (e.g. genetic defects) and behaviors easily avoided with proper training.
I've seen designer breeds that actually turn out for the better. For instance, Labradoodles were bred because someone wanted a better service dog that was hypoallergenic. With Labradors, you get the service dog trait. With Poodles, you get the hypoallergenic trait. A friend of mine has often chosen Labradoodles with good breeding as their therapy dogs, the ones that visit hospitals to comfort other patients, for such reason.
Now, as for the other wacky mixed breeds. I dislike people breeding them simply for monetary gain. So I'm not all happy when I read these postings online. As far as I know, no responsible dog breeder would post their ads online.