In many cases, it is handy to be able to specify an option in an alias, but still be able to override on the actual command line.
I can't say whether that is useful with this specific option, but it seems the same debate could be had about almost any option. I'm pretty sure the existing behaviour is consistent with other options, with the exception of a few (e.g. pg_dump --table), which are explicitly documented as being cumulative.
> Tatsuo Ishii <ishii@sraoss.co.jp> writes: >> While playing with pgbench, I found multiple "-M query_mode" can be >> set more than once. For example, > > I think that's true of just about every option in all of our programs. > Why is this one instance so much worse than others that it deserves > to be handled differently?
One of my colleagues actually believed that if both "-M extended" and "-M prepared" were specified, pgbench runs in mixture of those modes. So I felt avoiding such misunderstanding is more important.