Sure, we can still use gray cards, but they were for film, and histograms are for digital, and these should NEVER be assumed related.
Our camera meters are pretty dang good today, so the only purpose of metering on a gray card is to simulate an incident meter, to be independent of the reflected colors of the subject (these subject colors do cause grief for our cameras reflected meters). Because any white or reflective subject will underexpose, and any black or less reflective subject will overexpose. We have to know that.
If we photograph an 18% card, it should of course come out 18%, which is NOT the histogram mid-point. However all of our RGB data happens to be gamma encoded (entirely unrelated), which makes its histogram come out about 46% (and moves actual midpoint to around 73%). But the human eye never sees gamma encoded data (always decoded first, one way or another). If you expose a test to line up right at histogram 255, and then intentionally underexpose it one stop, it will NOT be 50% as expected of linear data, but will be near 73% (because of gamma). This 73% is NOT precise, because the camera is also doing white balance and contrast and color profiles, etc. But it won't be near 50%.
A few people today (even very proficient old timers) might calibrate their light meter so the histogram on a 18% card is set to come out middle 128. That's very wrong, for all the wrong reasons (gobbledygook, based on multiple unrelated and undefined meanings of "middle"), but is only about 8% wrong, not the end of the world, there are other variations too which hide it. It just sounds so dumb to say we did it. It's wrong to put it at 46% too, because camera is busy doing other things too. B&W film didn't do that, but color is a confusion.
We can still meter on a gray card for digital, same as we did for film, but Kodak always said to open another 1/2 stop if metering on their 18% gray card, still true. Incidentally, Kodak sold this printing business in 1995, so any card marked Kodak in the last 20 years is never from Kodak.
Today, some users use a gray card for white balance, but the cards are only spec'd to reflect 18%, and have no spec's to be accurate neutral color. Not greatly wrong color, but a real White Balance card will be better. 18% is too dark for WB anyway.
FWIW, Jones and Condit showed (1941) that average scenes had dynamic range about 160, and didn't mention 18%. Battleship Gray is about 40% (goal is to be hidden on horizon). Old printing shops did use a 18% card to judge their ink level of middle gray. Ansel Adams insisted on 18% as his middle Zone V (in the 1930s... Ansel likely never saw a histogram). Kodak never agreed with him, but in the day, there was no reason to dispute Ansel. :)
But light meters (and ANSI and ISO standards) are calibrated to work to about 12%. Sekonic says 12.5%. Kodak instructed to open 1/2 stop more if metering on their 18% card (which comes out 12%).
See http://www.bythom.com/graycards.htm and
http://www.ryanewalters.com/Blog/blog.php?id=3551583675371023276 for good info.