I think you'll find that (depending on the make/model) this "extra" becomes more popular as the idea of mini pci takes off.
wireless is moving quite fast - as is the idea of lappys as gaming machines.
BT have moved from 802.11b - n as their standard incredibly quickly - and manufacturers gain the advantage that they can be cutting edge without having to re-design the whole mobo.
It looks like the "standard" is or will be 2 slots, with network card in slot0
a network card consists of a number of devices - normally using a single driver - in this case they are showing seperately.