You're right. If all you want is something absolutely minimal, then that might work.
But having worked with clients for many years, I know one thing: It always starts with "Oh, we just want something very basic"
Next thing you know, they want to know how to accept one-off and subscription payments, automate content delivery, special member pages, protect files, broadcast emails, display teaser content for non-members, put login form on sidebar, and on and on and on.
Even without going into advanced membership plugin features like content dripping or upsells, I can tell you so many little things that they'll start wanting every time you show them something that gives them more ideas.
So, if your needs are really that elementary, then sure, like Christopher said, you can write some WP code and "git 'r done", like Larry the Cable Guy would say
But to achieve anything more meaningful, better to look at a proper membership plugin that comes with actual support, as you don't want your clients calling you in the middle of the night (if they already do, I do empathise with you :-).
- Ravi Jayagopal