My question is, why we should import free modern content which is well
hosted in repositories. Because we can simply do it??
I could say kinda the same thing for books that are well hosted in Google Books or Internet Archive...
The answer is that "wikisourcify" texts adds value. Otherwise, we shouldn't do that.
My reasons are:
* having these texts adds value to Wikisource as a library. It's high quality, academic and free content
** thus, it adds value to Wikipedia. Wikisource and Wikipedia will (I hope) grow in cooperation and crosslinking (wth Wikidata, for example). So having good content in Wikisource could results in having good content in Wikipedia (think about transclusion of quotes, or citations...)
* in Wikisource, you can add links to Wikipedia, or to Wikisource.
In this last example you can see how (via templates) you can create automatic category of citations (by and from)
* I like the idea of Wikisource as a "generalized library", a place for documents, books, texts, grey literature. I like these texts to be hyperlinked and connected, because we can do it, and we are the only ones in the we who can do it. That's a big reason for me.
* I am (as you are) an Open Access advocate, and I see this cooperation between Wikisource and OA books as important and possibily fruitful.