Crediting sources one has found citations from Suppose that I am doing a literature review on a topic, and I read paper A, which makes an interesting claim and references paper B as its source. Naturally, as paper B is then the primary source, I will look up B and possibly cite it in my own paper if it seems relevant. And suppose that it turns out that paper A says nothing else that's relevant for my topic, so I say nothing about paper A.

And this seems fine if I only discovered one citation from paper A. But if I find several useful citations in the same paper, it starts to feel like plagiarism if I just ruthlessly harvest it for citations without never citing itself. 

Sometimes it's easy to work in a citation in such a case. For example, if it's a review paper I'm reading, I might be able to just write in my own paper something like:


  Qwerty (2009) reviews a number of a results in this field, such as the finding that over 99% of the people who contract cancer wear shoes (Keyes 1999), that pink aliens from the sixteenth dimension dislike chocolate (Pinker 2001), ...


But often there isn't really any reasonable way to work in a citation, especially not if you wish the citation to make it clear that this is where you found useful sources from. I could include a footnote or a thanks in the acknowledgements, but I've never seen anyone do that for such a reason. Are there any rules of thumb regarding the correct etiquette in such situations?
The quoted you wrote is actually a pretty good way to include that citation.

It all comes down to the fundamental reason you are citing the paper. Citations are used to refer the reader to papers that contain useful information pertaining to your discussion. As such, “because it includes a good citation” is not a good reason for citing paper A in itself: there are plenty of other ways you could have found paper B (other citations, web or database search, bumping into its author at a meeting, etc.). Unless paper A adds relevant information to paper B, it should simply not be cited. One way it can add information is by performing a review of the literature, or by commenting on the results in paper B… In the first case, your quote is a good way to acknowledge that fact; in the second case, you could simply write that:


  Qwerty discussed in [21] the original results of Doe [22], showing how the data wasn't corrected to account for the phase of the moon at the time of measurement.


In conclusion, I'd say: determine whether paper A brings additional information, then either cite it or do not cite it. It is not common practice (and I don't think it should be) to acknowledge a paper without citing it.