Amazon is systematically adding service upon service to their offering. I'll be interested to see where this ends up once they're all tightly integrated and working together.
It's also an architecture consideration for the way we develop. Yes you want to use this Amazon feature, but if you do, moving to another provider becomes harder (e.g. we're looking to use SQS, but know if we moved provider I'd need to implement my own solution for that, probably mq rabbit).
To rehash a classic phrase: "Nobody ever got fired for buying Amazon."
Also, how hard is it to set up/care/feed a basic directory service? I've toyed with FreeIPA which was easy (http://www.freeipa.org/page/Main_Page, based on 389 DS). I actually did not realize Samba now includes a directory service and might check that out.
Setting one up is pretty easy. The problem comes in when you find yourself depending on it, and then it gets out of whack, or Just Grows and you've got OUs and CNs all over the place.
At my last employer - a mid-sized manufacturer - we had several Tier II apps. These must be in working order or production is shut down. The shop floor application. Core ERP. Email. Above II is I which are applications that must exist or the company cannot operate.
There was only one Tier I application: Active Directory.
That is you could treat AWS/any cloud as a distributed OS for clusters with VM/containers as processes/threads and services as packages.
It's more or less already roughly the case. Well, IMHO, it's the maturation path for the cloud I can imagine.