Regarding the 60D ... I found that I (me, personally; not you, them and others) derive absolutely almost zero benefit from the increase in megapixels, for the simple fact that, as an example, my 24mm lens cannot handle that resolution. So, whether I take a picture with my 24mm on the 30D (8MP) or the 60D (18MP), the result is very much the same due to the "lack of resolving power" by the lens ... and sometimes it's worse. This means an "upgrade" in camera also means an immediate "upgrade" in lenses as well.
I thought it was the case that the same lens will resolve more detail on a sensor with more megapixels, even if it isn't capable of resolving to the theoretical maximum of the better sensor? Isn't that in that hugely long equivalence article people link to occasionally? So the same lens cannot produce <i>worse</i> results on a better sensor (although a better lens may be required to get the best out of the higher resolution sensor)?
Output resolution is approximately the RMS of the input resolutions. That means that the lowest resolution component becomes the highest theoretically possible for output resolution. If you have a sensor capable of resolving 50lp/mm, then no matter how good the lens, your output resolution will never top 49.9999999...lp/mm. If your sensor is 100lp/mm, and your lens is really cheap and can only produce 80lp/mm, then upgrading to a newer lens would indeed help. Once you start approaching the limit of the lowest common denominator, it begins to take considerably more resolution in the other components to produce measurable increases in output resolution.
So, if you have a 100lp/mm sensor, and a 100lp/mm lens, and you get a lens capable of 200lp/mm, you will see an improvement. Use a 300lp/mm lens, and you'll still see an improvement, however it won't be as significant as moving from a 100lp/mm to 200lp/mm lens. Use a 400lp/mm lens (only possible at really fast apertures, like f/1.7), and you'll see a small improvement, but not nearly as significant as the others...and no matter what you do, your maximum output resolution will be limited to the sensors 100lp/mm.
You eventually get diminishing returns when using components with greater and greater resolving power unless you improve both. So, by the time you get up to wanting 200-300lp/mm (which can only be attained at faster apertures, like f/2.8 and wider), you should really pair it with a much better sensor.
But you are correct, at the resolutions of todays sensors, you can pretty much always see an improvement by using a higher resolution lens. And, conversely, moving to a higher resolution sensor will allow that lens to perform even better.
Regarding the 60D and the 6D ... I agree that both cameras offer spectacular improvements in high-ISO performance over my ancient 30D's and 5D. Only, I (me, personally; not you, them and others) don't photograph in the dark woods on a moonless night and, as stated previous, basically live at ISO200 most of the time. So what benefit do I (me, personally; not you, them and others) derive from "clean" ISO12800, huh? And only if the camera has a shutterspeed to match, which I (me, personally; not you, them and others) don't necessarily and artistically desire. But I've had this discussion before ...
For what it's worth, you don't need to be in comically extreme circumstances to need such high ISOs. Not that it makes a difference - you can work at 200, and so high ISO performance isn't important to you, as you say. But in overcast conditions around sunset, a bird - especially under trees, but even in the open - can easily need ISO 6400-12800+ - shooting at, say, 1/250sec and f/10. My point being, it's a fairly regular real world situation for some of us. Nighttime forests would require a whole lot more sensitivity (and I appreciate it was probably hyperbole, but I think a lot of people really do find it odd anyone would want clean high ISO).
Totally agree. It really doesn't take much to require high ISO settings. Especially when you need high shutter speeds like 1/1000s or 1/2500s, in order to freeze the motion of your subject (and flash isn't an option.)