Well, I don't have to imagine reasons to leave my wap off. I think I have them:

1. My house has CAT6 in every room on GBit router, and I prefer wired over wireless for so many reasons: one for all is performance (I have file server and network drives).

2. I am most of the day in the office, so does my gf, and my cats (in spite of being named Mega and Byte), don't browse the internet (yet). So, yes, definitely waste of energy.

3. Network Safety? Yes, I am paranoid. I could not care less of people stealing my bandwidth when I don't use it, but my home server has been hosting websites for years, so always on, and I don't like to have my WiFi interfere with it. Safest bet is to keep wifi off when not needed.

And, why not:
4. Do EMFs hurt you? Apparently not / officially not, but I always tell myself that centuries of science teach us that todays beliefs may very well be mistakes in the wiser eyes of tomorrow. So, since I only need WiFi to browse the internet away from my desk, here is one more reason why only then I want it on. Of course, there's so much EMF around us regardless, and this is really at the bourdary between science, paranoia, blind belief, and what not.

And, I think I can find some more reasons with a clearer mind.

Anyway, I am afraid I'll have to change my wifi habits as more and more devices move to wifi versus other radio standards...