I only left it on once. It wasn't just warm. I was somewhat hot. Not enough for me not to be able to hold it though. But still, I would prefer mine cooler than that. Whenever I see a use for leaving it on, I will consider.

I still think Nintendo should fix this and I hope it is possible through a firmware update. I mean, just turn it on once in a while.

By the way, a shorter lifespan is not so noticeable. Therefore, there would be the "lack of evidence", as you say it. My evidence is that it is hotter than it needs to be.

For future reference, mine is done in a heat wave sitting in 2nd floor of my house which is at least 5 degrees hotter than my first floor. My Wii was laid horizontally and after some test, I realized it's bottom was pretty hot (or at least warm) even with wiiconnect24 off... Also, I am not suggesting that WiiConnect24 is the actual cost, just that seems likely and better turn it off just as a precaution.