Hello again, hpa12.
I am using old primes on my digital body, some of which date back to the 1960s. They are more than merely serviceable for normal size enlargements. I suspect that it may be the same with the Rexatar, judging from what I am seeing and reading online.
The current generation of quality primes and zooms is in many ways a cut above most of the optics from that previous time. Better coatings. Computer aided design. Not categorically better in every way, but generally better, IMO. If one uses that old glass carefully and creatively within its more constrained performance envelope, I am of the prejudiced view that some quality images can be had.
Early zooms had some serious limitations, and which have been largely eliminated in newer designs. For example, my Nikkor 43~86 zoom is a nightmare in terms of pincushion and barrel distortion at the extremes of zoom. And it's inferior to modern zooms in resolution too. But even it is rectilinear at about 60mm. Modern zooms still have such distortion, but it is minimal by comparison. And now the electronic darkroom's editing tools allow for its correction as well.
By comparison, many primes of a generation ago still shine. I much prefer them for manual focusing, which by the way, is the only way they focus anyway. Some folks also like the rendering of the older glass with single coatings. As it is with film, lenses have personalities, each behaving differently from others in subtle ways.