At no point have I ever said you NEED a gray card to make correct exposures, what I said was a gray card would give you the most accurate baseline from the in-camera meter- which is a different thing entirely than saying you have to have it. However, if you're setting up for a shot in ambient light, and it's going to be a one-time action shot with a fleeting subject, then the second best way to get the shot exposed correctly is to set your exposure for the lighting conditions using a gray card and the camera's built-in meter[1]. If you don't mind doing more post-processing then the best way to get a raw file with the maximum dynamic range to adjust to the best exposure is the "white towel method" and chimping a UniWB histogram.
I do however completely disagree with your statements "it doesn't matter what your meter says" and "it shouldn't matter what the camera tells you is properly exposed." Both of those statements simply aren't good uses of the tools given. If it doesn't matter what your meter says, then why have a meter at all? If your meter isn't accurate enough to tell you what's properly exposed, then why do the manufacturers go through the effort of calibrating it to a standard? The issue with the camera telling you what's properly exposed is that you have to point it at a calibrated target to get it to do so- hence the necessity of knowing what a properly calibrated target is.
It still matters if you want accuracy- in case you're not aware what you get to see on the screen is a JPEG representation of the image, which may not represent the exposure as shot (that is, the JPEG is processed by the JPEG engine in the camera, not untouched.) Unless you go through loading up UniWB, your histogram is also biased on some channels. Of course, once you go to UniWB, you tend to lose the usefulness of chimping the image (though you gain more use from chimping the histogram.)
Actually, not many MF shooters shot 'Roids (I certainly didn't, and I'd say the number of people who regularly shot them was well under 20%)- roll film gives you enough frames to bracket the shot unless you're shooting panoramic formats- PLUS you have to be shooting a camera with a removable back- and I've owned five or six medium format cameras in the last 20 or so years- NONE of which were even CAPABLE of shooting a Polaroid.
Large Format is where Polaroid use was more important. I've owned 2 4x5s and 1 5x7 and shot hundreds of Polaroids. Bracketing a shot six ways on a view camera could take you 15 minutes, and by then the light could have changed and somewhere in there, you may have moved the camera, and with a Sheimpflug "wedge" of focus, that could be much more disastrous than with a simple SLR. Plus the 'Roid gave you a look at the overall plane of focus, which was more likely to be a big issue than exposure (you can always expose the first sheet and then adjust development and you can certainly adjust when you print.) Even shooting positives you could adjust time or temperature in the first developer after the fact, and you could dodge and burn if you were printing Ciba/Ilfochromes (been there, done that.)
Um, no- you're wrong. It is important, because it lets you know what exposure value your camera's metering is based upon. It's got nothing to do with blind trust- it's got to do with how the camera gives you an exposure value in any setting other than manual. If you only ever shoot on manual or you just take snapshots, then it's not important, otherwise it's important. Knowing the way the camera meters allows the photographer to know how much to adjust exposure based upon the metering target, lighting conditions, etc.
Why would one blindly trust the meter when it's simple to check its calibration? Of course, to check it- you'll have to know what it's calibrated to- and have the appropriate test target- oops!
In any case, knowing what it's calibrated to also allows you to also reference objects with similar values when shooting in the field. Half a stop is a 50% change in light- that's enough of a difference to not be "silly" to know where your baseline is. More importantly, choosing an exposure reference target in the field, you'll not want to choose something half again as dark as your meter's calibration.
In fact, the only time it's not useful to know where the meter is calibrated is if you're metering everything in a scene to choose a relative exposure based upon multiple spot readings of the light and dark areas, but in that case it's very important "what the meter says," since you're looking at relative values based solely upon those readings. However, that's a much, much less useful technique for digital and positives as it was for B&W negative film, since you're automatically discarding most of your dynamic range that way.
If you don't know what the baseline IS, then how do you know how much to adjust? More importantly, if you're going to do the work to use a card, using the correct card gets you to the right starting value- using the wrong one gets you to the wrong place pretty quickly. If you're shooting snapshots, it may not make enough difference- but shoot product for publication and you have to get the exposure and colors spot on or you can get to the "this product isn't that color" lawsuit land.
Besides, if it "doesn't matter what your meter says-" why are you using it as a baseline?
If you shoot with multiple bodies, knowing if there's an exposure bias between the two allows you (along with color profiling the sensor and manual white balance) to produce multiple images that match one another for look. That's why checking if your camera IS properly calibrated may be important.
Paul
[1] The most accurate reading is likely to be an incident reading from a hand-held meter if you're in a position to get one from the subject's location.
