The OP is bang on - I started work at Netscape in 1996, went over to Oblix in 1999 (A pure play LDAP management company) and then spent the next 2-3 years in a startup (Loudcloud) in which we tried to run our entire infrastructure off of Netscape Directory Server (Still, one of the finest engineered products I ever worked with - only had issues when you ran out of disk space).
Eventually, after I started managing IT, with all of the Windows Systems, and Exchange Servers, and Users - AD just infected us, and it's really hard to get out of your life. For a while, it was Blackberry Enterprise Server (BES), but being able to control whether people can login to their laptop/desktop/VPN/Email/etc... through AD is just so much easier when you have a lot of windows systems. Also - Group Polices, ACLs on things like Printers, File Systems, and other Resources - and now with Lync starting to pop up....
If you can live without Microsoft Exchange, and you don't have a lot of windows laptops, you can probably avoid it - but, AD really is the competitive weapon that Microsoft continues to be able to wield to keep themselves in the heart of a lot of IT environments.
Would love to go run an environment/company that had a pure-play LDAP server like OpenLDAP, Netscape/Fedora/389 Directory server, or, OpenDS.