Ruminate with me a while, won't you?
Ah, the Infrastructure Master. Probably the least-appreciated FSMO role of all. In discussions such as technical job interviews, most people can list the five FSMOs for me... maybe even tell me which are per-forest and which are per-domain... but if you then start asking for specifics about what each one of them actually does, the interviewee usually gets a bit more wobbly. And I think the Infrastructure Master in particular is probably the most difficult of all to grasp. I know it was certainly the last one for me to really "get."
I won't spill it all out here on what exactly the IM does - there's plenty of documentation out there if you're really interested. I would also direct you to this ServerFault post wherein I give a real-world example of what the IM does and what might happen if the IM is on the wrong domain controller.
This brings me to the Active Directory Recylce Bin. The AD Recycle Bin was introduced in 2008 R2, and was a long time coming. Before, restoring AD objects was a lot more arcane and cumbersome than it is with a good ole' Recycle Bin. Considering the AD Recycle Bin is going on 5 years old now, even though it's an optional feature, there's less and less of an excuse as time goes on for you to not have it enabled in your AD domain.
(You don't, do you?)
So here's the interesting bit that you might not have known: (sorry for wasting your time if you did know) once you've enabled the AD Recycle Bin, your Infrastructure Master no longer has anything to do. Nothing. Not even in an environment where some domain controllers are not also global catalogs.
When the Recycle Bin optional feature is enabled, every DC is responsible for updating its cross-domain object references in the event that the referenced object is moved, renamed, or deleted. In this case, there are no tasks associated with the Infrastructure FSMO role, and it is not important which domain controller owns the Infrastructure Master role.
So as the AD Recycle Bin becomes more and more commonplace in Active Directory environments, it seems that the Infrastructure Master may slowly dwindle away until only the old guard even remembers what it was, and budding young IT pros will only have 4 FSMOs to remember.