1. APU's you are right can only be paired with certain graphic cards however, that being said once you get beyond the 200 dollar range you will have far better performance with a discrete graphics card then trying to match it with a card to get there duel graphics configuration.
2. Its not ideal mainly because if you had plans to go with a discrete card you would go and get a different platform whether it was a AMD FX or Intel i5 you would get something else. They are directed at a completely different audience for people who just want a onboard graphics card and a system that works a very entry level gaming machine.