Liquid perhaps you were lucky, but a search on this site will show countless tanks that were nuked in a day by a heater malfunction.
Additionally, we're talking outdoors in Florida. We've had 95 degree weather the past couple months and it's going to continue for another month or two. We're also coming in to thunderstorm season. I've lost power multiple times in the past few months. So even if you had 10 heaters and 10 chillers none of that matters when the power goes out for a day. Of course Murphy says it's going to go out the minute you leave for work or go on vacation. So unless your going to buy a huge battery backup system ($$$), there's no way your powering heaters and/or chillers, as the wattage consumption are too great.
Conversely, lets look at winter in Florida. I'm confident in saying that when the power goes out and it's 30-40 degrees outside an outdoor tanks temp is going to drop extremely fast killing all of the inhabitants, especially any sps corals.
Finally, there's the high possibility of pollution. I thought about running my skimmer air line to the outside of my home to help raise pH, but after talking with some of the experienced LFS owners around here, the results would be negligible and only introduce the risk of contaniments entering my system. In fact, as I treat my property with pesticides or watch and smell my neighbors yard get bombed by Trugreen I always think to myself, I'm glad I asked and didn't do that. Now, look at a tank and sump and skimmer outside and think about Trugreen or any other lawn care company spraying liquid fertilizer and pesticides on a windy day and that contaminant getting into your system. It may be good if you have red bugs, but I guess at that point it wouldn't matter because everything would be dead. Also, the county indiscriminately bombs entire areas for mosquito control. I can't imagine that would be good for an outdoor tank either.
Again, it's a setup that is more likely to fail at some point so why bother. Now if you're willing to spend some serious money on life support, for example, multiple heaters, chillers and a battery backup bank on a transfer switch and somehow design it in such a way that pollution can't make it's way in, then perhaps the odds of success would increase enough to give it a shot. But after all the money spent on that setup, just build an in wall addition on your home for the tank. Then you can actually enjoy it during the dog days of summer and brisk winter days. I mean ultimately isn't that why we have a "display" tank to begin with... To look at it.
