Is "until juices run clear" a valid test for poultry doneness? Why or why not?
Chicken juices contain a soupy mix of proteins including haemoglobin (which gives blood its red colour when mixed with oxygen), and some myoglobin (which gives red meat its red colour when mixed with oxygen). Up to about 140F, they are unchanged, but heat them to between 140F and 160F and they lose their ability to bind oxygen and so their colours change. So if your juices are running clear, you know the temperature is at least higher then 140F and probably closer to 160F if they are indeed clear.

The question of what is 'doneness' is an interesting one. For most foods, doneness is a question of taste. After years of eating my chicken at 'at least 140F', I really like that taste. We usually cook foods to improve their taste, texture, nutritional value; very few foods traditionally kill or harm us if they are not cooked (cassava, certain beans are notable exceptions). Heating to 165F is recommended not for taste but to kill organisms such as salmonella - a tricky blighter that lives inside the cells of some other creatures and so can't be washed or peeled off.

I'm pretty sure that I'm the first generation cook in my family to own an instant-read probe digital food thermometer, so why is this now necessary? Well I may also be the first generation where salmonella in store-bought chicken is considered a saleable product and something to be cooked out by heat, rather than designed out by good farming and food handling practice. The 165F statement from food safety bodies was brought in to deal with such issues.

In France, there are still plenty of people who like their chicken very pink. With my own grown chickens, I will cook them till a skewer pierced into the deepest part of the thigh (but not near the bone), shows clear juices and on deep-breasted old breeds that can leave a tinge of pink towards the bottom of a (deliciously juicy) breast. I'm personally comfortable with that particular risk/reward balance.

But store-bought chicken? From an unattributed source? At $1:15/lb? Pass me the probe, please.