Here's an “against the grain” article in Scientific American about heirloom tomatoes found here. Interesting reading, but there several serious inaccuracies used as a foundation for their conclusion that heirlooms are weak. For instance “Any plant that sets only two fruits, as heirlooms typically do” – say what?? I've never read that, nor have I experienced it. Heirloom tomatoes couldn't have lasted this long in the wild if only two fruits were produced.
Nevertheless, it's worth a read. Check the comments for rebuttals.