USB v1 has 12Mbps max, shared among all USB ports. If This is the only USB port in use, that's fine, but if you have a USB mouse, USB printer, USB scanner, USB digicam like I do, if you try using them all at the same time, or even just a couple at the same time, your bandwidth to your internet connection could be inhibited. It also takes more CPU, as mentioned already. Also, NIC cards are one of the cheapest componants you can buy for your computer, so there really is no reason not to have one. 
Just my opinion.....
Cigar