I send a link to a friend saying "check out the kittens": <example.com/product/pr0n#/product/kittens> My friend ends up on a completely different page and there is no explaination as to why the page is wrong. Secondly, the URL leaked information about my browsing history. Finally, Google page rank goes to /produces/pr0n rather than the cute kittens.
If the URL is <example.com/#/product/kittens> then my friend sees the homepage rather than the correct product page. At least they'll know they're on the right site and can easily search for the correct page manually. Google page rank goes to the homepage rather than some arbitrary page.