Network cards are mandatory additions to PC purchases now that even the smallest
offices now have broadband Internet access and peer-to-peer networking.
Even if the cost of later purchasing Ethernet adapters a la carte were the same as the bundled prices, your clients still should get Ethernet adapters with their PC purchases.
The Benefits of Adding Network Cards to PC Purchases
When you add Ethernet adaptors to PC purchases, you and your clients won't need to worry about compatibility between them and other hardware components in the PCs and their chosen operating system. All device driver installation and configuration hassles are already taken care of for you.
You and your clients will also have an easier time getting help from the PC vendor's technical support department. The PC vendor should recognize the Ethernet adapter as part of the complete system and "supported" configuration. And, once again, the Ethernet adapter likely will be covered under any standard on-site system warranty agreement.