Above is an example of a Japanese advertisement for the American Product, "Vitamin Water," taken from their Japanese website. Although Vitamin Water is a loan word, roman characters are used, perhaps to maintain the familiar brand label. However, at the top of the screen, we see that the company calls regular water "ツマラナイ," or "boring." Because this is a Japanese origin word, we would expect to see that Hiragana is used; however, Katakana is used in its stead.
The fourth textbook example explains, briefly, that Katakana may be used for, "words the writer wishes to emphasize." This is supported by http://www.sljfaq.org/afaq/katakana-uses.html, which states explicitly that Katakana is used to emphasize important words, similar to the use of Italics in the English language.
These explanations make sense in context of the above advertisement. Because the company wishes to emphasize that regular water is inferior to their product, they will use text that reflects that emphasis. Imagine that the English website claimed that regular water is boring; you might expect their site to display the text: "Regular water is Boring," to represent the emphatic sound someone might place on the word boring when speaking it. The use of Katakana in writing ツマラナイ has the same effect, which is intended to convince the reader that they shouldn't drink regular water as opposed to vitamin water; in this case, Katakana is used as a marketing tactic.
The following image is taken from: http://www.takaratomy-arts.co.jp/specials/clockman/
In this example, we find two repeated Katakana words that appear to be coming from the alarm clock. These appear to represent sounds that the clock will make when the alarm sounds.
The second textbook example claims that Katakana may be used to represent sounds or "Onomatopoeic words." Another textbook called Let's Learn Katakana devotes an entire section to onomatopoeic Katakana words, and claims that setting these words apart from others gives them a distinct character apart from nouns, adjectives, etc..
This advertisement uses Katakana to lend the onomatopoeia a fun or playful sense to go along with the theme of the alarm clock. By replacing Hiragana with Katakana, the advertiser draws the reader into the fun and attractive sounds that the alarm clock makes, and making him/her want to buy the product.
In both of these examples, we find that advertisers use Katakana to their advantage; by using this text, they put emphasis or change the feeling of the words in a way that expresses something that usually would be expressed with tone or word order. Clearly, Katakana can be used not only for loan-words but to indicate a variety of spoken nuances.
Two of the four textbook examples (2 and 4) note that Katakana can be used for onomatopoeia, and only one notes that it can be used for emphasis (example 4). Perhaps because the most common use of Katakana is for foreign words, the textbooks wanted to focus on the most common interpretation and ignored the others.
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