The terminal block connector can be connected to a UPS, allowing redundancy in the power source: You can connect to a backup power source. This redundancy feature adds very little to the cost. We carry DC barrel connectors with lead wire.
Ubiquiti includes a 24-volt POE with all the gear that they ship with a POE in the box.
Power Over Ethernet was first launched by Cisco in 2000. The standards are IEEE 802.3af and IEEE 802.3at. "Midspan" means the same as POE injector: The term "midspan" refers to the concept of putting a POE injector as close as possible to the device being powered, since some voltage is lost in the ethernet cable.