As subject says, are they? As natural as we attempt to keep our aquariums why do we have to run heaters? Could it be expected that temperature wouldn't drop that much overnight as to overly affect things in the aquarium? Wouldn't natural bodies of water cool overnight just as an aquarium will?
Large bodies of water like lakes and rivers are not affected quickly by ambient temperature change. The amount of water is great compared to the surface area for heat exchange, so they tend to keep their heat. The change in water temp overnight is minimal. Fish tanks of a few hundred litres have a much larger surface area (and therefore large heat exchange area) and will change temperatures quite rapidly and to a much greater degree.
When I was keeping goldfish, the daily temp variation in winter was ~10 degrees celsius in my 100 litre tank. A larger tank may not be affected as greatly, but for tropical fish it may be quite stressful.
Of course it would depend on your ambient temperature. If your house is a toasty 30 degree celsius day and night (eg you have aircon) then you could probably get away without a heater. But if your ambient temp changes a lot , you'd be better off with one to keep the tank temp relatively constant.