Here is the deal
It bugs me that people can be so quick to disregard a canister, if that's what someone chooses to use.
People need to get over themselves and their high horse they rode in on.
There's obvious and not so obvious reasons to use a canister, and like
@simplicity stated they don't want to drill out the aquarium. Therefore the canister is a good idea to solve that solution.
Canister filter, HOB filter, sumps, sponge filters... they all work. Nobody is saying that canister filter wont work. I had a reef tank when a skimmer was basically an airstone. That worked as well.
The point is the drawbacks, the issues, the time consuming maintenance task required to keep up with these equipment. We try to mitigate all these things so we can enjoy our aquarium more. As you get into more serious corals and fish, the task increases and the enjoyment time decreases. We have redundancy after redundancy to avoid these issues.
I had a heater get stuck in the ON position and cooked my entire tank. I had my overflow tube get clogged by a suction cup causing my tank to overflow as the freshly refilled ATO kept dumping water into the sump. I seen a FX5 with a blown seal and half the tank onto a carpet floor. Even something as simple as a sponge filter airline hose siphon water out of the tank as the power went out.
We avoid these issues by installing heater controller or two heater controllers and setting a higher range for the second controller. We now have dual return pipes incase one gets clogged. We replace o-rings on canister filters before they go bad.
Nobody is saying a canister filter doesn't work or wont grow coral. I am just saying its more difficult and less attractive.
In my sump, I have about 50lbs of live rock, skimmer, Kalk stirrer, 3 part dosing, ATO, heater, fan, Nitrate brick, ground probe, auto feeder.
Now without a sump, I would have to find a way to store the additional live rock and keep its critter safe from being eaten by fish. Pump my Kalk solution into the DT or add kalk to my ato (which then has other issues). I will have lines and tubing everywhere on my main display where it would be visible. I am a neat freak and those tubing and wires would drive me nuts.
On my 750xxl, I have exactly 8 wires visible (all on the outside). The four MP40 and the 4 radion wires. No tubing. Very clean look