This link had some good arguments against exposing auto-incrementing numbers on a web page.
http://stackoverflow.com/questions/8078912/is-mysql-auto-increment-safe-to-use-as-userid
But generally, web developers feel uncomfortable exposing IDs that allow guessing other IDs by just incrementing or decrementing a number. Many resort to random, multi-digit IDs instead.
On a more obscure note, numeric IDs may also allow competitors to estimate your growth by keeping track of how the incremental value increases.