The International Cessna^® 170 Association

So what I'm reading into this is it's waste of time to lean
on the ground with a stock-engined C-170.

How often do you operate between sea-level and, say, an
airport that is at 7000+ feet msl elevation?

That's what I thought...

Is you're carb adjusted perfectly? For what altitude, what
ambient temp. and what pressure?

Yes, we have crude tractor carbs on our airplanes, but to
suggest that leaning on the ground is a waste of time is
akin to saying "the carb. on my airplane is adjusted perfectly
no matter where I go flying".

The fact is you can't hurt these things on the ground by
leaning (they'll fall on their face before anything bad
happens) and if you're like me, and you fly other types
(higher horsepower) aircraft, it's not the worse habit in
the world to aggressively lean during ground ops.

The only argument I can acknowledge against it is for
folks that forget to push the mixture full-rich before
take-off. To that, I say if you can't remember what
it takes to fly a given aircraft properly, well... maybe
you shouldn't be flying said aircraft....

(When I participate in on-going discussions of any subject in a conversation, I sometimes get criticized as appearing to "shout down" those with different opinions. It's not what I mean to do. I hope that's not how this appears. I'm only being conversational with regard to the subject. No intent on my part to overwhelm the opinion of others or to reduce participation is meant.)

Having said that,...

Bela, I agree and personally operate similarly. I operate each airplane as it's own characteristics and procedures warrant. If I'm flying a 414 or a Baron, I lean it at idle, especially after landing and during taxi-in, during the several minutes recommended by the mfr for turbo cool-down. Otherwise the plugs seem to load up and foul, and the next departure requires a plug-clearing runup. (I also refuse to shove the mixtures full rich prior to landing for the same reason, but I keep in mind the need to enrichen in case of go-around. Similar thing for the prop rpms. This type operating technique is generally reserved for the more accurately metered fuel injected, turbocharged installations, however.)
I am/was not addressing other airplanes in this thread. My comments were mainly in reference to the low-power, normally aspirated engines with their typical MS carbs and instrumentation that are normally found in our 170-types. And only with regard to taxying around at idle.

Agressive leaning at idle, in my opinion, regardless of altitude will primarily deny the secondary/tertiary idle circuits, which do not come into play until the throttle is slightly opened for acceleration. That's why the engine usually stumbles or dies when so leaned and acceleration is requested. (I guess it could be further posited that the most aggresive type of leaning might also get into controlling the primary idle jet, but that would be like trying to control the water fountain by partially closing the dam floodgates.) (Oh-Oh, now Noah ...er...Miles will have to bring me back to reality again. Lessee now, 4 fl. oz. subtracted from 14.3 million acre-feet of water, ...carry the 7....)
It really is an interesting excersize to actually measure the amount of movement the mixture control must make in order to affect the idle. From the full rich position, the control must be slowly pulled out, quite a ways relatively, almost to the cut-off positon. In fact, it IS actually at the cutoff position before any affect whatsoever occurs (if the throttle is completely closed.) If it affects idle before that point, then the throttle plate is probably not really set at idle position. And if THAT is the case, then the idle mixture screw probably IS set too rich....just to keep the thing running! I imagine that improperly adjusted condition would indeed manifest itself to an owner as a validation of leaning at idle. But as Miles and others point out, it can be done.
My opinionated suggestion is not meant to prevent anyone from operating as they wish. It may save a tablespoon of fuel over a very long taxi. (But then, a tablespoon is a tablespoon. Am I the only one who, ...having sampled the fuel sumps.... rather than toss it, ...go over and pour the sample into the tractor's fuel tank? And has anyone ever gotten in trouble at the airport for tossing it? Phoenix, Deer Valley has big fines warning-signs.)