Though there are certainly some *ir*rational reasons for IT
departments' behavior, there are also many rational reasons that IT
departments try to control the software running in their
organizations.
Condescendingly assuming that the IT department is run by idiots whose
decisions are ruled by emotional attachments (as one correspondent
suggested), or that they are irrationally prejudiced against free/open
source, and that it is obvious and irrefutable that you know better
than them (as was implied by some correspondents), may make you feel
better, but probably won't help much.
It also won't help much if you don't explain clearly and calmly *why*
exactly you need to use R for your work. You can use many kinds of
arguments, including technical (functionality, efficiency, capacity),
economic (no license fees), scientific-community (widely used in the
statistics community), and so on.
It *will* help to think a bit about some of the concerns that the IT
department may have. Many of these concerns apply both to free/open
software and to commercial software:
1) Security. They probably don't want you to install software which
risks exposing company data to the outside world either intentionally
or unintentionally. For example, they probably don't want you to run
code that mirrors your disk drive on an external server, even if it
claims to be secured cryptographically etc. Some companies will be
more careful, wanting to vet any software that can open a TCP
connection (which most non-trivial software systems, including both
Excel and R, can).
2) Protection against malware (also a security issue). Some software
which appears innocuous may contain a variety of malware. I'm pretty
sure that R+CRAN is free of malware, but I don't know what measures
are taken to ensure that.
3) Support and maintenance. Not only do they not want to be in a
situation where they're asked to support software they don't know,
they certainly don't want to be responsible for bad *interactions*
between your add-on software and the standard software.
4) Licensing. Besides the question of proper use of commercial
licenses, some licenses (notably GPL) have "contagion" clauses which
affect other software which is linked to them. Though this doesn't
affect the vast majority of users of R (because they neither modify R
nor redistribute it), your company's legal department will probably
want to know what's going on.
5) Interoperability, maintainability, and continuity. What happens
when the user of a particular non-supported software package leaves
the company or takes a vacation? Who is going to take over the work
he was doing? If s/he's developed programs/scripts on a non-standard
infrastructure to solve business problems, do the solutions leave as
soon as he's out of the building?
Even if the IT department *is* behaving irrationally, responding
irrationally yourself probably won't help your cause.
-s