Unless you've got some emotional attachment to a particular NIC, there is no reason not to use the onboard ethernet. Generally speaking, integrated ethernet offers less latency than an add in PCI card.
When it comes to onboard sound, even if you have 5.1 speakers, integrated audio meets all the average PC user's audio needs. Unless you need specific inputs, do a lot of audio editing, or demand absolute fidelity, then integrated audio is the best/most cost effective way to go.
For video, there are plenty of mobos that do not have integrated video. In some cases, like ATI's hybrid Crossfire or nVidia's hybrid SLI, combining an onboard GPU with a dedicated GPU has some advantages. Truth is, unless you are into gaming or video editing, onbaord video meets most/all the average PC user's needs.
As mentioned, regardless of whether the mobo has these devices integrated or not, you can turn them off or on at will in the BIOS.