Lets not trivialize a truly difficult problem just cause mongodb has some huge numbers on their marketing literature. Its naive to believe that scaling to the level of twitter is easy.

Sometimes you have to be innovative just to implement simple ideas in those text books or to make it actually efficient. Its easy to use type 1 UUIDs as ids but if its not practical to store 128bit ids for every value or you need to pack it into 64bits or need better ordering guarantees. you build something like snowflake.