So... what's the advantage to having a heritage breed? Is it just a purist desire or do they serve as a more functional breed somehow?
They aren't for everyone. Just as growing a garden and processing one's own vegetables isn't for everyone. Some folks are perfectly happy with quick, inexpensive, and frozen TV dinner type foods. It's fine. It's their choice.
The Heritage bird is TRUE to type, color, shape, and very importantly, true to personality that the original creators of the breed intended. They are often much larger, can be much more docile, grow slower, but live longer and have superior instincts for foraging and brooding (but not always) and simply ARE the birds that once were named, described and accepted as that breed.
The modern birds sold only "represent" those breeds. They mature quicker, usually smaller, sometimes lay heavier, but can often times experience burnout at the 2 to 3 year mark.
The experienced difference between having them, side by side, is often like night and day. Just my $.02