How detrimental is caseback pitting to actual wearability of a vintage watch?

Varies on the buyer.

Right, so this is common in vintage stainless steel watches, and tool watches that have been used, and if the previous owner(s) were blessed with slightly stronger sweat and didn’t really clean the watches. The watch is no longer waterproof, that is a given, but at this age it was never likely to be. It will be difficult to restore the case to anything like it’s original glory, but then you are looking at vintage with patina, this is not likely to be appealing. Can the watchback be closed properly?

Its a difficult one, because you are going to have to take care of such a watch regardless, but ensure it doesn’t get wet, and probably clean the watch after use / regularly to ensure no build up of sweat and grime which would continue the erosion process.

Thank you for your comment kind sir! I have yet to see the watch in person but I would assume that the caseback can be closed properly as the watch was recently serviced, according to the seller. Then again, after seeing your next paragraph on cleaning the watch regularly to avoid further erosion, it just does not seem worth it hahaha! Appreciate it always Chris👍

a lot of vintage is compromise, to some extent, as these things are 50-80 years old. I’m not suggesting giving the watch a sponge bath every day, but if you wipe off sweat and grime after use, and maybe don’t sleep in it, its going to prolong the life of the watch. 
At the same time, I doubt you could single handedly erode the watch to the point you structurally damage it unless you sweat pure acid. I have much worse watches in terms of condition, it’s kind of the charm.

I would recommend, for vintage Omega and other higher grade of makes, seeing the watch in person, and getting an understanding of service history.

I will echo the last point to see it in person, if possible. Pictures have a way of distorting defects and making them look worse than they really are.  But, camera angle can disguise a problem too. That level of pitting in a $20.00 Wyler wouldn't make me pause. But, guessing wrong with an Omega can really ruin the experience.

Excess pitting can effect the value & saleability as it will put some buyers off. The main physical issue is that the case won't seal properly & may let in moisture & possibly foreign matter as well

Hard to tell from the photo and the recent service might mean I'm wrong, but might that just be a melted or decomposed gasket?

Coyote Ugly is never that bad the night before. Think of how the watch will wear and present itself in the long run after you get it. 

Thank you for the further explanation Chris! I may need to be extra careful with these watches, I tend to sweat pure hydrochloric acid HAHA! 

Absolutely spot on, kind sir! I’m arranging to examine the watch in person, thank you for your comment! Appreciate it always 👍

Exactly! I may be one those buyers hahaha! And duly noted on the case sealing issues, thank you for your comment kind sir 🙏

I do not think so, although from what I’ve read, decomposed gaskets tend to contribute to severe caseback pitting. Thank you for your comment good sir!

I had to do a quick Google Search to understand that reference ha! You’re right, in the long run, I doubt I’m going to enjoy looking at those craters in the palm of my hand. Thank you for your comment good sir!🍻