Soup – Keeping noodles from absorbing all the soup

None of the answers so far mention collagen -- specifically, using a stock made from roasted and cracked poultry bones.

You don't have to roast the bones, but you do need to crack them -- use a large, heavy knife or cleaver (not a chinese vegetable cleaver, you'll screw up the edge) to chop the bones up into about 2" to 3" pieces (5 to 7cm). Put them in cold water, and slowly bring it up to a simmer, and just leave it simmering for a few hours. You can add vegetables, too, but you'll want to pitch them, as they'll have been overcooked by the time the simmering's done.

Then strain everything, and use that as the base of your soup.

If your refrigerate stock that has enough collagen in it, it'll set up like jello, and hold its own shape. Starches can work, but they're not ideal for soup -- flour (unless cooked as a roux) leaves a raw flour taste and cloudy soup; tapioca leaves little granules in there; corn starch will break down if you cook it too long.

similar to what julio said, the main reason is that typical wheat-based noodles release a lot of starch into the water, which changes the consistency of the soup. the starches can add a dirty colour to the water, but more than that, they can also thicken the soup undesirably (think of making a roux). Finally, if there are leftovers, the noodles can sometimes become completely water logged, making eating it the next day a bit of a soggy experience.

That said, with all these considerations there are times when you might add the noodles directly in-- non-starchy noodles (eg. like rice noodles) seem to do ok. also parboiling regular pasta before hand seems to help a lot with the starch and can still help you achieve the flavour integration you mentioned.