It's important for people to know how much their actual physical opens take:

• If they're using (only) an external pooler (e.g. pg_bouncer), they're doing physical open all the time
• They may have accidentally messed up their pooling config/connection string - seeing tons of unexpected physical opens would immediately make you realize that.
• If there are connection open errors, these will show up in tracing (though logging is more appropriate for this kind of thing)