[bdon]

It's also useful if you don't want your URLs to expose how many of a certain thing you have, whether it be users, posts, payments etc. A lot of sites let you derive how much activity they have based on how fast their numeric IDs increment. You could use a separate token alongside the pkey to do the same but this feature just makes it simpler.

[__Joker]

Along the lines

http://www.wired.com/autopia/2010/10/how-the-allies-used-mat...

http://www.jackofallclouds.com/2009/09/anatomy-of-an-amazon-...

[labria]

Only downside: this makes the URLs very long sometimes.