That's not what Puts says, nor what my eyes see. None of my '70s and '80s glass has light leaks during long exposures or wobbly front ends.
I've seen plenty of Nikkors with sloppy mechanics. My beloved 35/2 AIS was one of them. It was a fine lens for practical photography, though, and it served me well for 15 years.
The ZM light leak issue is not an optical problem, but a mechanical one: the placement of a screw hole. It is trivial to fix with a drop of black silicone, and it has not been noticed by most ZM users, many of whom are extremely critical. It is of course a real problem for some lenses used on some bodies. I assume that Zeiss and Cosina are looking into it.
Do CV even publish specs for the 50/3.5?
Not that I've seen.
Puts said it was better than the current Elmar, and Popular Photography said it was as good or better than any lens they had ever tested. In the post above, Tom A. says it's rather good, too. Say what else you will about them, but the Pop Photo lens tests were generally done to a very high standard, they still are, and Pop Photo tells you what parameters they are measuring. Importantly, Pop Photo still tests lenses directly on an optical bench, not lens-sensor combinations. Almost no one else does that, these days.
The LTM Hexanon 50/2.4 mopped the floor with the 50/3.5 when I compared them. The ZM 25 equal to a 28 Elmarit or M-Hex 28/2.8 from what I've seen, no better at all.
That's fine, but are you telling us something about the lenses, or about your own technique? I'm not just being cantankerous. If a given lens is not better for your work than another, you may value different properties than other people do.
What optical properties are important to you? Flare? Contrast? Microcontrast? Geometric distortion? Lack of astigmatism? Spectral transmission efficiency? Absence of coma? Speed? Compactness? Durability? Abence of color fringing? Performance at the corners on a solid-state sensor? Lack of focus shift? Sharpmess on center? In the corners? Absence of vignetting? Bokeh at wide apertures? At middle apertures? I am not sure which, if any, of these parameters go into calculating your "floor-mopping" score.
In any case, this has little to do with what I was getting at above, which is that the ZM lenses are not necessarily more expensive simply due to the presence of a blue label. There are good reasons to think that, on average, they're a bit more expensive to manufacture than the lower-priced C-V lenses, and their optical properties support this interpretation.
Whether those specific optical properties make any difference to your photography, or whether you personally should buy those lenses, are different questions entirely.