The only real practical advantage of dual NICs I've found (onboard or otherwise) would be to have multiple gateways. I've used this feature for Windows servers in order to get at multiple WANs or internet connections at the same time so as to avoid a hefty router upgrade. That's about it in my book.
Onboard 'giga-NIC' chips do take a performance hit over Enterprise class gigabit cards, especially the higher performing Intels. Also, the integrated giga-NICs on your typical Blade server motherboard are a substantial upgrade over your enthusiast Asus/Foxconn/Gigabyte, etc.
However, you aren't likely to notice much difference unless you are constantly moving big files that aren't already saturating your disk system I/O.
I have noticed that *good*, dedicated PCI NIC cards tend to be a bit more reliable than integrated NICs on enthusiast motherboards.