because 10/100 networking only uses 6 wires (3 pairs):
Correction, only uses 2 pairs. Orange/Orange White and Green/Green White are used for transmitting and receiving data. A lot of commercial POE systems actually send power over two pairs to achieve higher power transfer. As a telecom contractor, there were many times where I used the Brown/Brown White and Blue/Blue White pairs as either a second Cat 5 cable, a phone line or two, or to bring both DSL lines into the modem that we used for IPTV service. May not be the standard, but the standard doesn't work when you only have one existing Cat 5E and no options for running a new one.
That being said, always be aware of how the jack is wired on any device you are plugging into in the event that that device has the Brown and Blue pairs grounded or otherwise used. Last thing you want to do is short your power supply out! The CAT 5E standard allows for Blue pair to be used as a voice(standard phone), and you may find networking devices passing this through. Some people may see the possibility to use this as a potential power source for your devices while not paying for the electricity if you have a land line, and while this is possible, I would advice against it.