I was taught to do it, then I made libs myself where I've done it. Then I came across libs where it was one. Then I realised how clear and easy it was just to use one namespace. Now I use one namespace in my libs and only in projects where they fetish it I would adapt and do it.
A lot of answers advocating to do so are really opinionated and the reasons 'why' have more to do with clean working and organising ethics rather than being a clear advantage if you use folder = namespace.
Bottomline: meh! Satisfy yourself or satisfy others. Either is good as long as you produce clean code, architecture and be tight and organised. :)