While I do not classify it as freeze protection, I find (depending on the installation and it's application) It is usually slammed in when a restaurant finds out that the doors are impossible to open, pilots on oven equipment keep going out, etc... Outside air must be heated so the dang thing does not freeze out patrons and their space is having problems so in turn the duct heater is put in.
Now, I'm not crazy Eugene....lol. If this were a MUA equipment, then that heat is intended and sized for a specific application which may encompass exhaust fans and F/A make up to apply positive pressure to the space or structure. I think what he is seeing is a heater retro fitted to help out a problem and that was probably the cheapest answer to alleviate the problem. Yes, I know what you were inferring to and understand. I usually think commercial kitchen because I run into it all the time. Wow, I do sound crazy....lol How have you been Eugene?