The reason that AD is so needed is the ease of implementation with EVERYTHING, as long as its MS(ish) based. One AD account can auth a user to any service runni

The reason that AD is so needed is the ease of implementation with EVERYTHING, as long as its MS(ish) based. One AD account can auth a user to any service running on the company systems. For example, I work for a university as an applications dev; My local box login is the same for every VPN and additional university service that we use. Now, you can do this with OD, but it's not as easy as "built into all windows installations, set it once and forget it".