Having made stock and strained it, can the meat and vegetables boiled up in the stock be used for anything, or should they go in the bin?
I would dump them, with the caveat that some chefs reserve bones for making a second, lighter stock. I've only heard of this being done with Veal, so your mileage may vary.
I've also heard of people combining stock and chicken salad making by throwing a whole chicken in to the stock pot, then using the white meat for salad.
This offends me on many levels, but I'll just say that it probably makes for bad chicken salad, and also almost certainly makes for cloudy stock.
I actually Googled this myself because I was wondering about the same thing. Someone else also suggested pureeing the veggies and adding them to spaghetti sauce. http://vegetarian.betterrecipes.com/vegetable-puree-leftover-vegetables-after-making-vegetable-stock.html
If there's any flavor left in chicken meat on the bone, or much collagen left in the bones after stock making, I'd say it hasn't simmered long enough. The stock should be both flavorful and unctuous, which means the meat will be flavorless and the bones brittle. Overnight is great for this, at a very low temperature - but with aromatics added later.
If you boil your stock, the tiny orifices in the bones from where collagen leaches will be sealed off, and the result will be much thinner than it could otherwise be.