When poaching chicken (whether in soup or in some other liquid), you need to be careful not to cook the chicken too long. Overcooked boiled chicken will be flavorless and have a bad texture.
For chicken breasts, if they're whole, don't cook the breasts for more than 10 to 15 minutes at a simmer, although you can leave the meat in the liquid for another 15 minutes so long as the heat is turned off. Here are some recipes for poached chicken.
On the other hand, if you're trying to make chicken soup, you're going to want to cook the chicken much longer, but that meat will be fairly inedible. By simmering chicken on the bone for an hour or two, you'll get a nice flavorful stock, but you'll overcook the meat.