I looked around for an interesting phrase that would be interesting to analyze on a blog where others are required to comment. In a short student paper posted online, I read that Japanese users not only borrow specific words, but they combine words into phrases that would otherwise have no meaning or different meaning in the original language. The example that I found most revealing is ケーキカット which is adopted in mainstream culture to mean the ceremony of cutting the wedding cake. In English, we do not designate a specific term for this activity, so even though this is clearly a loan word from the English words "cake cut," this phrase has no significance in English. I would categorize this type of loan word as having "Slant Meaning." The original meaning of the words are kept, but a derivative of them is used in Katagana Japanese.
In contrast, a loan word that doesn't fit into "Slant Meaning" is マックフライポタトthat means McDonald's French Fries. Even though it is not a direct transliteral loan, as it comes from Mac Fry Potato, the meaning is more or less direct.
A more simplified or more direct loan would be ブラシ in which both the transliteration, sound and meaning are transferred.
Naturally, kanji would not be used to express these meanings as not only wecre these concepts first introduced to the Japanese in a non-Chinese language, but they are used phonetically in the spoken language, and so translate well into either hiragana or katagana.
So, why katagana? In Chinese there are several radicals that demonstrate a foreign or borrowed word, item or concept. These radicals signify concepts or phrases from the West or from the ancient North, and might have indicated the import quality of itself in order to differentiate between Chinese culture from "other" cultures at a time when the state of China was struggling to unify itself as a political entity, socially distinct from other cultural peoples. These other cultures might be Mongolians, nomadic "barbarians," Europeans, Buddhists and Persians. Perhaps, the Japanese, after adopting the Chinese language in the Tang Dynasty adopted this tradition, and has continued it through katagana instead of identifiable radicals.
Another possibility is that after Japan's defeat in World War II, its government instituted a series of nationalistic measures that sought to "erase" or minimize China's historical influence on the making of Japlanese language; and thus, the shaping of its history. Katagana could be used to add a layer to Japanese language that doesn't exist in traditional Kanji - phonetics, calligraphic style and short history.
Katakana textbooks are brief in their explanations of categories because katagana changes quickly, and textbooks are supposed to be used for many years. Although each text sticks to more or less one or two general ways in which katagana are used, there could be multiple reasons with no real rules to follow. Some of it does seem a bit on the arbitrary side, as it's a conglomeration of colloquial usages. Thus, the textbook cannot accurately capture all of its uses without confusing the reader. As a student of History, I am most equipped to give a historical analysis as opposed to a linguistic or cultural one.
It's interesting to see these three levels of loan words coming into play in Katagana. Language is fluid enough that each generation has a new set of vocabulary, connotations and denotations. As the original English or Chinese meaning is distaanced from users, alternative meanings are attached for a new effect. The purpose is to use existing vocabulary, and give them new meanings in new contexts for a different effect. This is one way in which language captures the evolution of a culture.