The Nikkor 35mm-PC was my workhorse lens back in film days. But with digital, I wonder if there is really an advantage with using these lenses for perspective correction?
It depends on the actual lens, but I'd say that with the latest lenses, yes there is an advantage. But wait, there's more ...
You can easily correct keystoning, etc in Photoshop (not to mention other lens flaws like barreling and vignetting. I know that this results in some loss of IQ from the pixel interpolation involved, but what I recall with the 35mm PC was that you could notice some image deterioration as you approached full shift, so that seems to be a wash.
Maybe. Part of the complication is with Photoshop, since it's not offering the best resampling quality possible. Depending on the image content, you may find more choices to adapt to image content in some dedicated Pano Stitching applications (they may, besides keystoning correction, also offer geometric distortion correction, and a choice of (higher order) resampling methods), even on a single image. Anyway, software will not solve the need to crop after resampling, which might reduce the resulting number of pixels to output.
Not using any PC-lenses these days, I'm curious to know what and how great the advantage of using optical correction as opposed to digital correction actually is (I guess I'm talking about the 35mm FF world, here).
Not unimportant, lens quality has improved. This raises the stakes for the resampling alternative. Also important, the wider the field of view, the more optical detail counts as an advantage over software/interpolation. Also, the more extreme the shift, the more the optical detail advantage weighs in.
So, it ultimately depends on the actual situation, which lens, how much shift, which focal length. With a good lens, optical quality will probable win, unless the amount of shift gets relatively extreme. When the software alternative is chosen, stitching with a (slightly) longer focal length may offer an even better quality, although it requires more postprocessing time (and some skill/experience).