"Katakana are not used only for foreign words. Katakana are also used with onomatopoeia, newly coined words, names when you don't know the kanji, and more.
Furthermore, katakana can be used to vary the style in writing. It is not uncommon to see an entire sentence in nothing but katakana, often to express a certain emotion (even when it lacks foreign words altogether). In other words, ヲ is actually used once in a while.
In addition, in ancient times, katakana were considered the proper way to write the language, while hiragana were considered sloppy and vulgar. In this respect, katakana can be used to evoke an archaic style. "
Not to dispute your (very credible) assertion that it's more suitable for common use than hiragana, can katakana express more tonal subtlety than hiragana? Are there words for which it would be preferable to use hiragana for legibility reasons? Are both used together in compliment to express an even wider range of verbal tones?
Yeah... about that. Do you really need two different phonetic alphabets? I mean, c'mon! Give a lazy gaijin a break!