2. If you click it and are expecting "product A" and instead you see "product xxx" you probably won't proceed further.
3. This is Amazons fault, not those that have posted short URL's. There simple is't a longer URL that says: http:// amazon.com/this-is-the-product-you-were-after-just-in-case-you-are-skeptical-of-a-short-url-given-to-you-byu-someone-you-dont-know-beware-danger-will-robinson.
a.co links always go to some illegible Amazon URL, so I think it’s fine to use them.