Anyone who argues otherwise, is simply wrong, they are entitled to be wrong, but just don't follow their blind ignorance, your family will appreciate your vigilance.
Huh?
I argue otherwise. Engine monitoring systems are a nice to have, and very handy for diagnosing some engine problems in very rare occasions. Diagnosing engine problems is great, if you get it early, and if you're in time to fix early. For that, I support their use - if the pilot knows what they are looking at. But if that problem has already shown symptoms in flight, you're also having to deal with it and fly the plane, perhaps as an inflight emergency. I have certainly flown with pilots of abnormally well instrumented aircraft, who were paying so much attention to the indications, that they forgot to fly the plane - and what they were seeing was really of little benefit to them anyway by that point.
All certified aircraft have engine operating instructions - just follow them! Keep the engine properly maintained, and serviced with the proper liquids, operate it as instructed by the manual, and it will run. The few pennys you might save by extreme leaning and other tactics are lost on a carburetted engine, because as Big Pistons correctly points out, no carburetted engine has fuel flow even enough to each cylinder to run "properly" leaned - that's just the way it is, and you the pilot cannot change that, so just pay for the fuel, and fly the plane. The only way a scanner can help with leaning is to assure that you are leaning to the leanest cylinder when reading the indication, and allowing you to find a power setting which has a more equal fuel distribution than other power settings, so your leaning is more meaningful. In reality, however, the generic "lean 'till first rough, then enrichen to smooth again" instruction works about as well as watching a scanner for 10 minutes while fooling with the mixture.
I have installed scanners for certification flight testing, where data collection well beyond the scope of any GA pilot was required. Two were for detonation testing - that's a whole other discussion, other than to say that if you operate as instructed, you will not detonate the engine. Incorrect leaning of an otherwise properly operated engine can cause damage, but not because detonation is a factor in that damage.
Both my planes do have multi cylinder scanners (left over from testing), and both are carburetted. All the scanners tell me is that I don't really need them 99.95% of the time! I do not peak, or lean of peak lean, as both engines prohibit it (same logic as not over speeding or over stressing it). Their only real benefit is that you can log a changing trend - if you notice in time, and you can identify which cylinder is sticking an exhaust valve or fouling a plug, in real time, and that's informative on the very rare occasion it happens. Though if you stick a valve, and have only that cylinder repaired, you're fooling yourself (and your family may thing you not vigilant!). Other than that, the operating instructions for the engine describe what you need to know to operate it the way it was approved. What else were you thinking to do?
My guidance is not blind in saying the forgoing, as I am delegated to certify the designs of these engines, including detonation testing. I write operating and maintenance manual supplements. The first thing we look for are safe operating margins, and instructions, which if followed, keep the engine well within those margins. People choose a certified aircraft [and engine] for the security of knowing it has been thoroughly tested. It has, so operate it the way is supposed to be. I am unaware that any aircraft has been certified as requiring an engine scanner as a part of it's type design - because they are not
really needed!
If someone wants to install one, by all means. Don't let it distract you from the safe operation of the aircraft. Use it to follow (not circumvent) the engine manufacturer's operating instructions. And learn to use it properly. Most flying schools probably struggle to employ instructors who are conversant with the different scanner types, their relevance, and their correct operation. And students hardly want to spend an extra 5 hours of flight training costs to learn what it is telling them, and the relevance.
My family appreciates that I keep myself current and recent, and that I operate the aircraft legally, safely, and in accordance with it's instructions. More than that, they do not ask of me....