Comment by SigmundA

I have found very minimal penalty on secondary index reads in practice such that it has never made a difference.

Remember some databases always use clustered index internally (SQLite, MySql) such that even if you have no primary key they will create a hidden one instead for use with the index.

https://www.sqlite.org/rowidtable.html

It is nice to have the choice which way to go and would be nice if PG implemented this. It can have significant space savings on narrow table with one primary index and performance advantages.