I've been using cheap little Shurflow type pumps for a few years now.... and a cheap temperature control, to turn on at thaw and off at freeze... No valves or heaters, etc. etc.... and I just leave it out there. Have never had a pump freeze, that I'm aware of... I suppose there's a possibility that it starts up when froze and thaws itself out...
But bottom line.... It has been working just fine to leave it out. Haven't had to replace a pump yet.