Just throwing an anecdote out there, but my company uses UUID for primary keys on every table in the DB. While int4 is for sure more popular, it would be nice if there weren't even more reasons to "force" people in that direction. I know I started regretting the decision to go with UUID primary keys slightly once I realized that we'd need exclusion constraints, and you have to jump through hoops to use them together.
My main point is that maybe the reason why most users use int4 pkeys (besides conventional wisdom) is because it gets the most support from features like this, and it may not be quite as skewed if that same support were given to other types.
Just my $0.02
-Adam
