Keep in mind,
suexec does not eliminate all security problems, and deals only with a very small subset of issues. You mention that a hacker could still compromise your site, but as a different user--which is true. But consider this--on a shared hosting environment, where all the Apache instances are running as
www-data, an exploited Apache process now has access to everything
www-data has, which likely spans multiple users.
So if you had a user that could ONLY view its own files, had a jailed shell (if the hacker was able to exploit and login as the user), disabled login, etc., then exploiting that user, specifically, would have only a limited effect. The purpose here is not to prevent hackers from getting in, but to limit their damage once they are. Since CGI can be exploited, it still falls on your shoulders to make sure that your scripts are still secured.