The one problem with using your distro's Postgres is that your upgrade routine will be dictated by a 3rd party.
And Postgres upgrades are not transparent. So you'll have a 1 or 2 hours task, every 6 to 18 months that you have only a small amount of control over when it happens. This is ok for a lot of people, and completely unthinkable for some other people.
Why would your distro dictate the upgrade routine? Unless the distro stops supporting an older version of Postgres, you can continue using it. Most companies I know of wouldn't dare do an upgrade of an existing production database for at least 5 years, and when it does happen... downtime is acceptable.