Did all of those diesel engines have working glow plugs? As for working in the rental equipment business, as if any rental company in the history of the world has ever given a crap about the longevity of the equipment in their fleet. It gets used, abused and then sold off with all manner of tax write-off advantages.
Your "personal opinion and experience" don't mean a thing to me and my equipment. I've started cold diesels thousands of times in cold to very cold weather both with and without the use of a block heater. I know which is better and which both I, and more importantly, the engines prefer.
"a waste of time and money" .... seriously?! A block heater is cheap, electricity is cheap, rebuilding or replacing components that have failed prematurely is not. Using a block heater makes batteries last longer, starting motors last longer, glow plugs last longer, engine components last longer etc., but feel free to do whatever you want, just don't pass on your ignorance along to others since that sort of thing is highly contagious.