Somewhat related to T192526, I think it would be helpful for readers to have the same behavior with regard to references in Wikipedia articles, so that people copying and pasting passages from them wouldn't end up with e.g. "The quick brown fox jumped over the lazy dog."
It's not like this is a bad idea – it isn't – but considering some more arguments I have to agree with what @Legoktm said above. But I'm just another dev with an opinion. I feel like this is something that needs a UX and/or product owner decision, despite being a rather tiny detail. @ovasileva's team maybe?
@BrandonXLF very nice that you've already included a patch, thank you for looking into it! +1 to @Legoktm's hesitation however, I don't see a community consensus yet to make such a broad-reaching change, only two people and the task author have joined in so far. In the meantime, this patch suggests that the desired effect can be achieved with a user stylesheet.
Would it make sense to propose a user preference for this behavior?
The concern about attribution is worth considering. I don't think it's really feasible to try to block people from copying from Wikipedia, though, even if we wanted to—we'd have to try to disable copy and paste entirely to do that. Our license indicates that text drawn from Wikipedia must be attributed, and beyond that, it's on users to make sure they use the ability wisely.
To clarify, my comment wasn't about attribution. It was that people like me prefer to have the reference numbers in the output because it indicates that the statement being copy-pasted has some reliable source/reference behind it, which is what the xkcd comic alludes to - if the text has  in it, it must be true ;)
I, too, would side with @Legoktm on this one. Those users who prefer the ref numbers not to be copied can always achieve this using a user CSS like proposed in the patch. But the default behavior, IMHO, should be to copy the text as is.
Note also that <ref> tags are not just used for references (in the bibliographical sense of the word) but also for footnotes, etc. When copying text like "Foo bar loerm ipsum[footnote 1]" it actually might be desirable to copy the footnote reference, so as to note forget to also copy the footnote later on.