I have a big 48 port PoE switch (Ubiquity) and power tons of devices from it. You can get a GbE PoE “uninjector” which will give you a 5V micro USB output and use that to power tons of things all over the house, while also leaving the GbE usable at full speed (on Amazon). I also buy PoE versions of things whenever possible. I even have PoE switches powered by the core switch and home automation gear powered by PoE (e.g. Control4). Great low wattage DC distribution, and getting greater with PoE++ (I have just one 4 port PoE++ for now, but it is also 10GbE.)

Can't carry enough current to run more than some electronics though. PoE vacuum cleaners wouldn't work.

True, but you could certainly charge a roomba over PoE.