heat increases resistance, so as the wire heats up the resistance will increase. As resistance increases voltage will drop if the current stays the same. Not only that but it really depends on what you are using it for. If you are powering a light, fan, or something trivial then who cares. If you are running something with electronics than that can spell disaster. Also if you have anything with a relay/contactor that added heat can cause the relay coil to heat up, start chattering, or possibly start melting.
Basically whatever you use it on will lower the life of that product due to added heat and stress on electrical components.