I have used it many, many times, backing it up with a half hitch, for climbing,
I am glad you have survived the use of the square knot and not had an accident. I am quite serious and I am not trying in the least to be funny or sarcastic.
I looked through some of my references for mention of the square knot, so I could offer something besides my own opinions. Three of my more modern references didn't even mention the square knot; another only listed it in a table, offering that a square knot reduced the strength of the rope to about 43-47% (compared to 80% for a figure eight). Finally in a 1942 publication I found the following "For joining two ropes a fisherman's knot...will be found better than a square knot... and is easier to undo afterward."
The generally preferred knot for joining two ropes together, as say to establish a rappel, would be the double fisherman's knot; some use a variation of the figure eight for this application. I would not quibble at seeing a double sheet bend applied to this purpose but I would never put body weight on a rope joined with a square knot.
I began technical climbing around 1957, later including vertical caving and technical rescue, being particularly active from around 1965 to 1985. Even now as a geezer, I still do the occasional rappel to retrieve a juicy paleontological specimen. In all this time, I have never heard or read of anyone advocating use of a square knot for any technical purpose, with the single exception of the quoted reference above.
What I have observed over time is the greater detail paid to knot tying, the increasing use of more secure variations of knots (such as double loop bowlines for the standard bowline), the better analysis of accidents, and the universal precaution to back up every knot tied with two half hitches or the equivalent. A lot of this has to do with the now universal use of synthetic materials in climbing ropes compared to the manila or hemp ropes in use until after World War II. Some of the more recent synthetics (Spectra and Dyneema) don't knot well at all and must be sewn or knotted with a triple fisherman's knot - the characteristics of the rope must be considered when deciding upon the appropriate knot to employ.