Because if you put the $90 into a better card you in most cases would get more
That's a silly argument.
1. The discrete card will require more equivalent power than the nearly free hashing power from the integrated GPU.
2. The discrete card will use up a valuable pcie slot on the mobo (if you are going for full density.)
3. On normal rigs the CPU slot is wasted by an inefficent SLA-256 calculator. The APU remedies this.
4. All other things being equal, a rig built with the APU will outhash the same rig built with a standard CPU.
There really is no argument.
Spending $90 more for 60MH/s is silly.
Your point 1 is wrong. You are not getting free hashing power. You are overpaying for the 60mh/s AND paying more electricity as the APU uses more power then a CPU with the same power. And you are using 50 watts over a sempron 140 that would make the most sense in mining.
$90 will buy a 5770 that will do 1/3 more hashes then the APU
going from a 5830 to a 5870 will cost about $90 and give you more then 60 MH/S
going from a 6950 to a 6970 will cost about $90 or less and give you more then 60 MH/S