In general I agree that rechargeable 9V batteries are not worth the bother in the usual battery-operated smoke or CO alarms.
However my US location has a continuing problem with at least a dozen power failures a year. The power people tell me that is because of old buried wiring in the neighborhood. My house has 3 AC-powered combo CO/Gas alarms, 2 with LED display, 1 with LCD display, and with a 9v backup if the AC mains fail. Usually a normal alkaline 9v battery will discharge during the length of an AC power failure. So to solve that steady waste of alkaline batteries, I use the Maha 9.6v LSD batteries. Although they cost ~$10 each, over the years, those rechargeable 9.6v batteries have proven to be cost effective. and yes, if the 9.6v Maha battery ever runs down, the low-battery warning does sound, even for a day if we are out and about.