A decent NIC has onboard RAM and Processing capabilties: it can lower latency and increase thorughput , The same goes for a qulity HDD controler whether the drive type be SCSI, SAS, FcAL, IDE, or SATA a decent Controller with a dedcated IO processor and ram can speed up Hard drive access and transfer. Two good examplse of how onboard stuff is usualy junk are video and sound, because most of us can directly tell with our eyes and ears the differance one video or sound card can make. Same is true for other intergrated stuff that at one time required an aditional add on card (IDE and Ethernet).
PCI- express should render Intel CSA's dedicated bus nearly obsolete, since a single PCI-e lane has enough bandwith to handle GIGBIT network traffic; But I don't expect it to disapear any time soon, since it is cheap and Intel's marketing Dept loves it.
By the way, I think the markting dept @ intel dictates (or does) all the engineering. That would explatin all of the problems they are haveing with their products and crap they are putting out while making such huge profits.
edit:
Realtek makes some of the crappiest onboard lan (and NIC's for that matter), they often poop out and quit when they are under full load, resulting in some type of crash.