I was not talking about POE particularly, just rigging your own POE not using injectors. The UPS protects the computer and camera in the event of a bad AC spike, surge, etc. Because if the computer gets destroyed its likely whatever is connected will be affected also. Additionally, the in-line fuse on the 12v line can protect either device depending on which one fails. A power supply does just that -regulates and filters AC with a rectifier and produces DC power. If you know what you're doing (and hopefully make it look nice) theres no problem at all tapping your power supply for voltage. Again, "if you know what you're doing"