Honestly I wish more people would do this. There's no such thing as being too safe. Especially since smoke detectors are a dime-a-dozen. There is really no reason not to have one. No one needs to wear a seatbelt until you get in a wreck. As far as light pollution goes, 99.9% Of smoke detectors only blink for 1 millisecond once every minute or so to let you know that batteries are still sufficient. The light given off by a smoke detector won't even come close to harming your plants during their dark cycle. I say do it and I would love to hear any arguments against it.