I have that identical heater and find it useful but not essential. I also have a quality bedchair, 5 season, fleece lined bag and a Trakker peachskin over blanket.
There have been times when I've opted to light the heater an hour or so before I intend to retire. Then get sorted for the night session, into the bivvy, switch off the heater and take full advantage of a warm space, aided and abetted by the items mentioned previously.
Mostly I don't, though.
Ian