Jump in a Beijing taxi and show the taxi driver the pinyin for the address you wish to go to instead of the Chinese characters. Report back on how well it works out for you.

One reason is ambiguity, this is an extreme example but shows a bit of the issue: http://en.wikipedia.org/wiki/Lion-Eating_Poet_in_the_Stone_D...

A lot of people have asked this same question for a long time.[0]

[0] http://en.wikipedia.org/wiki/Simplified_Chinese_characters#B...

They provide a significantly greater density of conceptual labels, allowing better mental separation of the homophones.

Different dialects of Chinese use the same characters. Cantonese, for example. Switching to pin yin would only work for mandarin.

Disregarding your question, which others have answered plenty well already, I find it funny that you refer to them as hieroglyphics, since actual Egyptian hieroglyphics ended up being discovered to be phonetic as well!

partially phonetic

Because each pinyin could mean a thousand of different things, so it's way easier to understand what they mean with an ideogram.

Chinese writing does not encode sounds. It encodes meaning. If you write Chinese in pinyin, all you do is write down the sounds, not the meaning.

Said differently: why do we keep writing English like this, instead of using a phonetic alphabet? Same reason.

Some of the characters do actually encode sounds, typically in the right side of the character. However, the encoding is more like a memory aid than it is phonetical. 中 (zhong1), 钟 (zhong1), 种 (zhong3/zhong4); 艮 (gen3), 跟 (gen1) 根 (gen1), but 很 (hen3). And it doesn't always work: 立 (li4), 位 (wei4), 拉 (la1).

Not everyone knows and uses pinyin, especially older people.

They are not hieroglyphs, you will see no pinyin publicaly in China. Many ignorant americans come to China having studied Pinyin and they fall flat on their face.