One of the acceptable reasons for deleting an answer is "This answer is a link-only answer". From this, I imply that we desire to have some exposition and should not simply referencing a scholarly work without any further discussion or introduction.
I see little difference from a link and just a quote with little or no accoutremonts. The only additional value that a commentary (only) provides over a link is that it does not suffer from link-rot. If link-rot prevention is the only reason we do not wish to see link-only answers, then simple quotes should be considered a good answer - but I believe there are many more reasons for not providing a link-only answer.
For example, Jon Ericson pointed out that answers should
add some sort of value over what might be discovered via a search engine
quote only answers dismissive of the question in the same way that RTFM is. If the question can be answered with a quotation that is easily discovered via Google, the question was probably not constructive in the first place. So if another answer covered the same ground without being a bare quotation, the question must have had some merit.
But most of all, this site is designed to provide expert answers. Presumably, if the answer is already readily available, people would not be asking here. We are to be experts, not merely quote experts. Answers should be innovative and provide new, additional and supplementary material. If a question could be answered with a quick and basic search, it probably wan't a good question in the first place.