It's your car; drive it how you will. But yes, any action which increases engine speed and load via gear selection will result in additional stress, strain and wear. The extent this will manifest in reduced engine life, increased oil consumption, transmission failures, is unknowable, depending on the robustness of the design of components. (This discussion has been with us since the dawn of the automobile. I still remember a girlfriend whose first car was a Renault Caravelle. Her father proudly told her she'd been given a sports car and taught her "Never lug the engine. Downshift for corners. Use the redline. " That little POS was totally toast in 10,000 miles of doing exactly what she was taught. A male friend bought a Ford 390" 4-speed; he always drove it as hard and fast as it would run and never had a problem in 100,000 miles.)
Bottom line: Do it your way. I have a good friend who's a serious hypermiler; has driven Honda Accords for many years and they're still like new when he buys another. He states proudly he's never ever had to use full throttle upshifts or cornered hard enough to wear the little rubber bumps off the side of the tread on his 45 PSI tires. The idea he'd downshift for a corner would horrify him.
Good advice for less gas used and less engine wear is, IMHO, in order of relevance: drive less, anticipate the road ahead and drive gently, in traffic go with the flow, and when you dictate the pace, don't speed and don't drive at the car's limits. Use or avoidance of engine braking is far down the list, making whether it is a pro or con mostly moot.
We can also say that driving spiritedly, while fun perhaps, causes more wear and uses more gas. But if we want to downrate our own pleasure and uprate component wear and fuel used, we should avoid getting into a personal car at all. We should board the bus.
Notwithstanding all of the above (i.e., whether we should even care) the question of whether engine braking is good or bad requires thinking through all that goes on in situations where we need to slow the car.
If you see a red stop light ahead, the best thing to do is start gently slowing the car (on a good day, the light will change to green before you get there and you'll never need to completely stop at all). Let's consider a few approaches to that slowing process, assuming a manual transmission car:
- No engine braking. Push the clutch and gently press the brake. Engine revs will drop to about 1000 rpm (reducing, but not eliminating, wear) but the car will be burning fuel. There will be wear from combustion (explosions!) happening in the engine, wear on the fuel pump, and wear on the brakes. There is some transmission braking because the car is still in gear, and some wear on the transmission from being spun by the wheels (the clutch disconnected it from the engine, not the wheels). Because the engine and the wheels are disconnected, if the light goes green you'll need to change gear to a lower gear and release the clutch, causing at least the normal wear of a gear change. Failure to rev-match will cause additional wear.
- No engine braking, no transmission braking — use neutral. As above, but rather than hold the clutch while braking, pop the car into neutral and release the clutch. This will reduce wear on the transmission (which will otherwise still be spinning while the engine idles). It will cause the wear of changing into and out of gear (as well as the wear from allowing the transmission to come to rest and then spinning it back up to speed if the light goes green before we get there).
- Engine braking. You take your foot off the gas without pressing the clutch and the drag of the engine slows the car. Perhaps as the car gets slower and engine RPM drops below 800, you need to change down. Engine revs average higher than option 1 (increased wear), but no combustion is happening in the engine and no fuel is flowing (reducing wear). No brake wear occurs. There will be wear on the transmission from changing down. But if the light goes green, you're already in the right gear, so you can just smoothly (and gently) accelerate.
I think most hypermilers would use the third option because it avoids needlessly wasting fuel while the car is slowing. The question of what causes most wear to which components complex. Does idling an engine at 1000 rpm cause more wear than freewheeling it at 3000 rpm? Is it more important to carefully rev-match your shifts than avoid changing down for engine braking? etc.
Finally, is the person who has a 5 mile commute to work and drives it spiritedly putting less annual wear on their engine than the person with a 10 mile commute and drives it like they have a glass of milk on the passenger seat that they don't want to spill? We don't just have the added miles, we also have the fact that the sprinted driver, by staying in lower gears and using more power, may be warming the engine up quicker.