This is usually site to site differing. I think DSP.SE allows this, where as in the Programmers.SE it is considered not-constructive.
This looks like an apparent contradiction but the utility might change. In case of DSP, one is usually referring to set of papers - and doing google search for papers even if easy, only when someone has read some of these paper himself/herself does it make sense to recommend. No one really reads tons of research papers unless you are following some citations or special recommendations - so this makes perfect sense here.
In case of Programmers.Se -if i ask what is your favorite book on design pattern?
and you know you are now beating the Amazon's recommendation engine. It's just waste of time there. Though it is not completely banned there as well, it depends on the questions.
So essentially, for every site value of these references do make a difference.
In case of workplace, i think we are dealing with generalized subject matter even more than programmers; so allowing "my favorite books is ____" will collect huge crowd but not value.