As far as the low/low-mid end discrete graphics is concerned, the main advantage they have is probably not allowable power draw. For portable systems it's the total system power draw that matters anyway, and even for stationary systems there are advantages to being able to power manage CPU and GPU in a coordinated fashion with APUs. For a given cost of power supply and a given level of system noise, an APU will probably be able to be fairly competitive in terms of computational power, (and superior in terms of needed volume, potentially allowing some innovation and diversification in form factors).
Rather, the main advantage with a dedicated GPU lies in it having it's own, fast, memory pool. For instance, even today some HD5550 cards have 128-bit GDDR5 interfaces, giving them some four times the theoretical bandwidth of the upcoming APUs from Intel and AMD, where additionally the main memory interface has to be shared between CPU and GPU needs. That's a major differentiator.
Also, since the PC processor business model hinges on being able to sell the same or similar part at a multitude of tiers, it is unlikely that the main memory channel will move very far from the low end CPU needs. Indeed, AMD seems to have no interest in offering anything beyond 128-bit DDR3 for their upcoming processors no matter the price tag, and Intel will only offer 256-bit memory on their highest end processors, and those who are prepared to pay for that are unlikely to want that bandwidth shared with on-die graphics. So, for the foreseeable future, bandwidth is the argument to be used to promote stand alone graphics.
Rather, the main advantage with a dedicated GPU lies in it having it's own, fast, memory pool. For instance, even today some HD5550 cards have 128-bit GDDR5 interfaces, giving them some four times the theoretical bandwidth of the upcoming APUs from Intel and AMD, where additionally the main memory interface has to be shared between CPU and GPU needs. That's a major differentiator.
Also, since the PC processor business model hinges on being able to sell the same or similar part at a multitude of tiers, it is unlikely that the main memory channel will move very far from the low end CPU needs. Indeed, AMD seems to have no interest in offering anything beyond 128-bit DDR3 for their upcoming processors no matter the price tag, and Intel will only offer 256-bit memory on their highest end processors, and those who are prepared to pay for that are unlikely to want that bandwidth shared with on-die graphics. So, for the foreseeable future, bandwidth is the argument to be used to promote stand alone graphics.