You have no idea how many times I've had to help people who punched their own cables and ran them 11223344, rather than 33211244. Sometimes it works (short cables have no problems), but when you run 100 feet and don't pair up #2 properly, you end up with a cable that looks good on a simple tester but doesn't actually work when you actually plug it in to a computer and network switch.
What I never understood was why Ethernet uses the pins 3 and 6, rather than using pins 7 and 8. Keeping the center two pairs for telephone lines would have made it dead-simple to use the same jack for both purposes. My office, for example, has 3 identical 8-pin jacks under each desk. One is used for the telephone system, and the other two are used for PC's. The context of each jack is based on where the wire goes at the patch panel in the equipment room.