Head on over to techPowerUP to see their review of the FSP Twins 500 W Redundant power supply. If I remember correctly, this is the same PSU that Paul over at the [H] recently reviewed.
There are probably some users with deep pockets who don't mind investing a significant amount of money into a PSU with redundancy a normal ATX chassis can accommodate. Those are the users FSP had in mind when it came up with the Twins line idea. Although 500 W may look like very little, it is enough for a low TDP CPU and a new graphics cards, given both feature low power consumption. Such a PSU isn't there to power a strong gaming station, but a PC that needs to be accessible around the clock with as little downtime as possible.