Things should have the minimum permissions they need in order to operate. Tradeoff between usability and security. As the example of permissions also shows, there's always a tradeoff between usability and security. The windows in your house make it more usable, letting light in and letting people see out. But it's easy to break a window to get in.
Some people put bars over their windows.
Some build concrete houses with no windows at all. Then again, sometimes people put up bars and then leave the front door wide open. Whatever you do, you need to evaluate the tradeoffs involved and make conscious decisions about them. It undoubtedly did make his system easier to manage - but he wasn't thinking about that tradeoff with security when he did it. As of version 8.9, sendmail is paranoid by default.