Can I add uncooked noodles directly to soup?
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.