Two things others haven't mentioned:
Interoperability with other companies or the public
This is particularly true with consulting companies, but any company could eventually have to release documents to the public.
If companies you partner with are using different products than you, then you may have incompatibilities. This is mostly going to cause a problem with office software, where you send them a presentation, a spec, a spreadsheet, and they can't open it, or it looks ugly. One way around this is for everyone to standardize on the same software, and the easiest way to make sure you have the same software is to buy the version that has massive entrenchment.
My pet peeve here: PDF are not standard or the entrenched leader, even though a lot of people think they are.
If you are working with Visual Studio, you probably bought MSDN subscriptions, which means you probably get everything else on the Microsoft stack along with it. This is as good a reason as any to use all of what you've purchased. You'll be stuck with that software forever, and have added resistance to change it, but that's one reason that companies go all-Microsoft.
I've seen similar negotiations for Flash, Flex, Adobe, and HP products (since HP has the "official" test/project management suite for Flex development).