In my experience there's probably a case for both. Having a local IT person you know and trust to take care of your immediate needs is going to be beneficial but it would also be nice if that person didn't have to patch your email server personally. Decentralized IT is great for things like email and shared hosting and generic services but individual teams may be better suited to having more direct control. I don't think its an all or nothing situation.
It actually IS nice when they have to patch your email server personally.
* They're ALLOWED patch it. When the org gets too big, any patch is going to break someone else's workflow. Everything freezes into place behind change management by people who don't know the full system for people who are too busy to read every sub-organization's change management announcements.
* They WILL patch it. They know you, your boss knows their boss, you all matter. In a large company the individual asking for a change is an idiot, and annoyance, and a nobody- no matter how smart the change is.
* They CAN patch it. In the small IT org, there isn't room for someone who doesn't know what they're doing. In the large IT org, there is room for enough people who aren't technical to hide out they can start hiring their friends and crowd out all technical skill, while maintaining a thick smoke screen that diffuses all responsibility.
This really runs counter to my experiences in IT. Generally the larger the company I have been in the more likely my email and printers actually work.
Front-line IT is much more about soft skills than technical skills. Putting paper in printers and showing users how to use google are not hard technical skills but are crucial to the success of an IT organization.
The people who understand how to manage email servers should be doing so for the whole organization because supporting 10,000 users isn't really much more complicated than supporting 10. There's no reason to have 100 email administrators in an organization when it can be done with a handful.
The change management procedures for sub-organizations are irrelevant, the marketing team just gets email from somewhere, they shouldn't have to know or care where it comes from or how it is managed as long as the SLA is met. Let them carry on with what they are good at instead of struggling to find a competent email administrator.
I agree the team that manages email should be small, just like the team that manages the printers or whatever other IT plumbing but to me that's the benefit of a large IT organization, instead of having to find one guy who is a jack of all trades (and master of none) you leverage economies of scale and get experts in each service to manage them for everyone.
