In my workplace (c. 750 desks) we have standardised on Windows XP for non-technical users.
Technical users can use any OS they like as long as they support it themselves and they sort out the licensing. Quite a few tried Vista, but I think they're all back on XP now.
We find that there are a bunch of advantages in standardising and very few disadvantages, but perhaps I'm missing something and we'd be better off with a large number of Macs and Linux users and four or five Windows versions.
If you are standardised, what OS have you standardised on - and how big are you; I expect that the experience of a 10,000 desk organisation on NT4 will be very different from a 20 desk organisation on Mac OS X 10.5.
The main pro for a "standard" desktop OS is maintainability. It is much easier to get to know just one OS and adjust to its strenths and weaknesses.
However, how long are you planning to stay with that single OS? The market will move on and newer things will come out and there will be need to change. So, a single OS can only work for any given time.
Just because Vista proved too much effort for many people to switch to will not mean XP will live forever.
Having a look at alternatives does not hurt. It could provide alternative ways of thinking and thus allow a new perspective even on a tried OS way.
BTW: you did not provide any detail of what it is you are doing on those computers, so it is rather hard to provide an answer for your particular situation.
As you have to support the systems (I presume?) the less amount of applications/systems you can get away with, the better (cheaper, higher quality) aso). One desktop operating system is enough work really, unless there's some specific needs (like QA testing, but they should probably use local virtual machines that isn't supported by the internal IT ops).
Non-technical users most likely won't care much about the version of say Windows, so stay where you feel comfortable. Use a license which gives you the right to run any version and try to upgrade every 2nd or 3rd year if that will please the majority of users ("cool, new stuff!").
I.T.: You want to make your life as easy as possible, but remember your company is about getting their work done. Given the premise of the technical users, you should be able to have your way with 95% of the desktops, and let the "Mavericks" do their own thing.
For most office workers, the choice of OS and such, is not that big a factor.
Users: You can run your own computer, but remember that I.T. isn't going to be able to help you if you get into "interesting" kinds of trouble.
Personally, I have a 98 box I use to get work done. Then I also have an XP box onto which all the "enterprise" stuff goes.