Using anything for something outside it's intended purpose typically voids the warranty as well as any protection you may be entitled in the event that it malfunctions and causes a fire or injury. So "it isn't designed for it" is a perfectly reasonable excuse not to do it in my book. That being said, your animal's safety is your responsibility, so if you feel comfortable doing it than that's your choice. I think Caitlin is just trying to offer a safer, longer term, solution to your issue. I use UTHs in almost all of my reptile enclosures, all controlled by thermostats, and I've never had an issue. The horror stories you typically hear, come from people plugging them directly into an outlet and just letting them run.
A quick Google search turned up this Article from May of this year that doesn't recommend sleeping with an electric blanket on all night, or for a prolonged period of time.
https://www.mattressadvisor.com/safe...nged%20period.