Spaces carry a lot of semantic weight, as do other components such as capitalisation and punctuation (most of which in use in Japan is borrowed from romaji anyway).
Well, in English spaces certainly carry weight, partly because the word unit has so many characters. In languages like Chinese and Japanese, units /are/ the characters (or a few of them). Particles then make natural separators in strings of kanji.
That's exactly why you need more than just hiragana, it'd be too difficult to tell what each character was parf of at speed. Something like katakana replacing kanji and hiragana for particles would be perfect.
Modern Japanese could learn a few lessons from hangul imo.
iimagineitdgetprettyannoyingtryingtoreadenglishifitwaswrittenlikethisbutitshardtotellwithouthavingdonesofortwentyyears