The number of bugs in a chip is relatively proportional to the number of transistors - Bob Colwell, former Intel chief architect

You will notice that between the two 16x slots on this NF590 chipset mobo, DFI put in two different sockets. Instead of two 1x slots, this DFI has a 4x and an 8x, a potentially useful and fun setup. Actually, the 4x only has 2 lanes running into it, but that is not a huge disadvantage.
If you want to run a decent RAID system on your SLI box, or you want to do multiple GigE channels, you probably would welcome the extra bandwidth. Even if you only have a few 1x cards, the slots are still backwards compatible, so there is absolutely no harm in having them there. In doing this, DFI made a pretty unique board with only up sides, almost no down sides, and a negligible added cost. µ