Re: There should be an UUID class (feature proposal)
One reason is space. A string representation of UUID is 36 characters long (with dashes), and the number it represents is 16 bytes long. Obviously, it's very unlikely that one would run out of disk space on your database server because of that. However UUID columns are often indexed, they can even serve as table's primary key. And for indexes, size matters. The bigger portion of index you can keep in memory, the better it performs.
Another reason is database-agnosticism. One can use the uuid type in config/schema.rb, but then this file won't work with other databases than PostgreSQL. I see no good reason for that.