Great strides have been made towards improving yields and disease resistance. With recent advances in biotechnology we even have genetically engineered marvels like cotton that can be over-sprayed with roundup and corn that makes its own insecticide. Many modern varieties are hybrids. If you save the seed from a hybrid there's no telling what you'll get, but chances are it wont resemble the parent plant. Heirloom varieties usually can't compete with the hybrids when it comes to yield, but I have to believe that they were saved for a reason.
Heirlooms are the plants that our forefathers survived on.
They may not look as pretty, ship as well, or have the same shelf life, but every heirloom has some unique or interesting characteristic which is responsible for its popularity, and hence its survival. Whether it's 'Rutgers' tomatoes, 'True Hubbard' squash or 'Danver's Half Long' carrots, heirlooms offer something a little bit different. Many people like the way heirloom's taste, but perhaps the best thing about heirlooms is the story behind each variety. Howling mob corn was named because every time the producer pulled up at the courthouse to sell, there was a howling mob ready to buy his corn. Moon and Stars watermelon was introduced by the Henderson Seed Company in 1926.