Standard ATX is 7 slots... even if a board only has 6, 4 or 3 slots. The board will still fit in the case. And as others have said, those extra slots come in handy for some boards or accessories that USE a back-plane area. Like additional USB ports, Audio, control switches. Most people are not using all the slots anyways and its a non-issue. But if some gamer has a 3-way SLI board + audio card, that alone could eat ALL 7 slots. And then they'd may need 1-2 additional slots for those accessories.
Its possible thare are server/workstation boards that can use upwards of 10 slots... but I doubt that such boards exists. With onboard controllers, and onboard 2~4 NICs, SCSI controllers - there is little need for such huge boards.
Besides... that Thermaltake case is a gamer case. And if you look at the inside view, the bottom drive-bays wouldn't allow a board that actually uses 10 slots anyways.
There is a better and cheapre case I'd recommend:
http://thermaltakeusa.com/Product.aspx?C=1274&ID=1415#Tab2 Its about $250 and it rocks...
