This keeps being spewed around as if miners are just blindly buying ineffective hardware.
The fact is that the 6990 is a great miner because:
1. It has two GPU's in 1 card
2. If your motherboard has 2 PCI-e slots, you can turn them into 4 gpu's worth of hashing power.
3. If your motherboard has 4 PCI-e slots, you can turn them into 8 gpu's worth of hashing power.
In the case of single core GPU's, you would now have to buy 2 computers with 2 power supplies, processors, motherboards, RAM, casing etc.
So in the end you save space & don't have to buy a second rig to house more cards & you get greater resale value.
This keeps getting spewed around because its true for most people. For the price of one 6990 without a new system, I can get an entire pc with 3 5850s that have better cooling and 50% higher hash rate. Thats only for the 6990 (580 euros minimum). It doesn't matter how many spare slots you have on your mobo, at 750MH for 580 euros, it simply cannot be justified. At all. Not even by power consumption. If you are considering spending 600 euros on mining equipment, I suggest spending 50 more, and buying 2 mobos, psus, cpus, hdds, and 4 5830s or 3 5850s and get 50% more hash rate at only 50% more power consumption.
Also consider noise. If you put a 6990 in your living room/pc room/bedroom, it becomes uninhabitable. A separate rig can be put into your basement or an unused room. Heat generation can also be a problem. My 2 rigs are in a guest room with full open windows 24/7. 3 5830s and a 6950 mining 24/7 generate around 35C ambient in the room, with outdoor temp around 20C. I don't care because I'm not there, but back when it was in my room it used to be really hot.