heya. i'm not too sure myself but my Japanese teacher said the distinction makes it somewhat easier to spot. it eases the flow of thoughts or something like that. just like how Kanji is used to sum up a whole chunk of hiragana so it makes things easier on the eyes as well as the mind.
personally, i've seen Japanese girls writing 'Happy Birthday' in hiragana instead of katakana, so i suppose there isn't much significance in differentiating the two. heh.