I once had to use a paper from 1953. I only found a few pages, it was a Polish translation (which neither me nor my teacher can read) and I had given up when I found it by an absolute coincidence on a website in Spanish dedicated to something else. I still decided to quote it because I was really proud that I had found it.
The teacher said we should all have done that, even though I told him I had no idea what it said, I had just used the picture with the name of the species underneath, it was almost impossible to find and the wikipedia page had the exact same info, only... available.
90% of the reason they don't want you to use Wikipedia as a source is so you actually study the subject of the paper rather than just reading one brief article on the subject. The other %10 is the lack of reliability. It is a good place to look for sources though.
The teacher said we should all have done that, even though I told him I had no idea what it said, I had just used the picture with the name of the species underneath, it was almost impossible to find and the wikipedia page had the exact same info, only... available.