Why did PostgreSQL merge users and groups into roles?

I found this thread in the PostgreSQL-Hackers list, from June 6, 2003, that in the end suggests that users and groups and roles be consolidated. (Thanks Craig Ringer for suggesting that I check the pgsql-hackers list archives.)

Here are some benefits mentioned (those that I found).

allow groups to have groups as members

the ACL code would be simplified

the GRANT/REVOKE syntax and the display format for ACL lists could be simplified, since there'd be no need for a syntactic marker as to whether a given name is a user or a group.

In some circumstances I could see it making sense to allow logging in directly as a group/role/whatchacallit

This would also solve the problem that information_schema views will show only owned objects

[makes it easier to] representing privileges granted to groups [since you'd simply reuse the role related code?]

The merge has many advantages and no disadvantages. For instance, you can now seamlessly convert a "user" to a "group" and vice versa by adding / removing the LOGIN privilege.

ALTER ROLE myrole LOGIN;
ALTER ROLE myrole NOLOGIN;

Or you can GRANT membership in any other login ("user") or non-login role ("group") to a role:

GRANT joe TO sue;

You can still:

CREATE USER james;

That's just a role with login privilege now. Or:

CREATE GROUP workers;

That's effectively the same as CREATE ROLE now.

The manual has it all.