Focusing Attention Without Drawing Attention
The goal of the practice of typography is to present text in its most effective way. Anything that distracts from the special space a reader occupies when submerged in a text is a disservice to the reader, even though the reader may not be aware of exactly why a text is more or less enjoyable to read. Harmony of presentation is the key, and sloppiness or inconsistency with spacingbetween characters, words, or even among paragraphs or pagestend to "misplace the attention of the reader," as that old version of the Chicago Manual of Style succinctly puts it. The conscientious typographer, Chicago continues, "scrupulously avoids any typographical mannerism which might draw the reader's interest from the text to the vehicle which is conveying it." This includes intrusive typographic styling as well as sloppy typesetting.
This doesn't mean that text type has to be boring, and I subscribe to Frederick Goudy's avowal in his 1942 book The Alphabet that "no one can look at an early printed book without feeling the beauty of the type page, for the old printers' types were inspired by the letters of the handwritten books, and with these for models they played endless variations on the alphabet, while our present types in the main are absolutely monotonous, with no artistic flavor or thoughtfulness."  It is no doubt true that the exigencies of cut-rate printing technology have favored typefaces lacking in finesse, detail, and character.
Typeface choice for text is a thorny issue for just this reason, because in general, ornament is at odds with readability. More commonly, the issue is not one of ornamentation or decorative pizzazz, but of simple appropriateness. As Walter Tracy summarized the issue in his book Letters of Credit, "Set a novel in a newspaper type and the effect will be so uninviting that the success of the work will be jeopardized. Set a newspaper in a book type and we will not take it seriously."  He went on to cite the case of a newspaper using the book face Caledonia, winning design awards but soon sliding into bankruptcy because readers found the type too odd. It poisoned the message.
Despite Tracy's sage advice, Times Romana classic newspaper faceis the standard typeface for paperback novels. It's chosen for the same reasons Morison designed it for The Times of London: It sets compactly (saving paper) and prints legibly under the cheapest printing conditions and on the worst paper. In a book, however, it makes for gray and joyless reading.