On getting a controller I quite agree. It's cheep insurance to a failing heater, plus most of them have digital displays so you can see the tank's temp at a glance.
As a historical note. Heaters haven't changed too much in the last 50 years. You'd have thought that by now every heater being sold would come with a controller. Controllers today are very reasonable in price and would solve a lot of the problems we see in typical aquarium heaters.
The other area where I see a lot of people get into trouble with heaters is that they way over size them. Unless your keeping the tank in an unheated basement or garage you only need about 2w per gal of water to maintain the tanks temp. Also too large a heater will do a lot more short cycles giving it a lot more chances to fail.
This has been one of my more nagging items with the hobby. We all see the ratings and they are often used to size our heaters. But then when I look at the how and why heaters fail and stick on, it really makes me wonder about the ethics of companies who insist larger is better.
I do a lot of "autopsies" on failing equipment and much of it involves electro- mech stuff so tearing down a failed heater just comes natural for me. Here is my spin on some often overlooked points about sizing heaters.
What makes the "click" we often hear when heaters cycle? It is some form of contacts closing. In the cheapest type it is just a bi-metal spring which warps as the temp changes. In more expensive it can be a relay closing. But in both cases, there will be a tiny set of metal contacts slapping together and pulling apart. You can see a small arc as this happens if looking at some glass heaters. Every time this arc happens, it makes a tiny burn on those metal contacts. As the contacts get older the burn gets bigger. The mechanical sorts that work on older cars with points in the distributer know what happens when that burn spot gets big enough. They weld themselves together! The next step is the tank overheats.
So if we put a really big heater in a small tank, the thermostat will turn the heater on/off much more frequently making those contacts burn much more quickly than if it is a small heater that takes much longer to do a very gradual temperature rise.
So just to use random numbers, you may get a 300watt heater operating/burning those contacts 4 times and hour where a 50 watt may only operate one time and stay pulled for much longer. Since heater failures are most often in the controls rather than the heating element, small heaters that come on and stay on will tend to last longer than high wattages heaters.
We all can agree that we need enough heater to maintain the tank temperature in that room under the normal temperature but given a chance, try using the smallest heater that will get the job done as it will often last longer.
Like incandescent light bulbs, they don't normal "burn out" but fail when turned on or off.