If we ignore the part of recommendation questions leading to opinion-based answers, the main concern against these kind of questions is that they don't add anything of value to this site.
But also one of the main design concerns of SO: they didn't want to end up as yet another low quality programming site with broken links all over the place. That's why they always had a policy against "link-only answers". In addition, link-only answers open up for spam.
10 years of SO experience has shown that almost every single question asking for off-site resources or recommendations is also low quality in itself. The rules against off-site recommendations and link-only answers served that site very well.
I see no reason why we should have a different policy, it will only lower the quality of the content. We must have some sort of quality of content concern on this site, or it will end up as yet another trashy programmer forum like so many others out there.
If needed, we can always put together a maintained list of external resources, book recommendations etc inside the specific tag wiki.