If that's true then why do people keep quoting it as the reason for not using open source software that has no legal entity behind it?
Because corporate companies want to deal with corporate companies only. That's why Red Hat has made a success in the gnu/linux world. On the other hand, Ubuntu has helped gnu/linux to be a more widely known OS.
Coming to
drhowarddrfine's OP;
Another reason I can think of is that Microsoft provides integrated solutions, such as Exchange Server + LDAP Authentication + Lync Communicator (and this can be integrated to desk phones), www proxy etc... companies do not want to bother with "hacks" to make these ecosystems work seamlessly. These systems/software may not involve in "mission critical" tasks, but lead "microsoftication" (I made this up, but you know what this means?) of the whole system.
I work for a telco operator which provides wireless and DSL solutions to customers. Most of our OSS (Operation and Support System - they are critical systems, one OSS goes down, then you have no mobile coverage/DSL service in one part of the country) systems run on UNIX-like systems (there are some SunOS systems, as well as Linux machines)
Big enough companies (Netflix, Google, Yahoo) who have enough IT/engineering resources definitely choose any UNIX-like system (name it linux, FreeBSD, or any commercial UNIX), or develop their own OS based on one of the FOSS OS'es. But mid-small range companies rely on "turn key" solutions, and at best make a support contract with an IT firm, that's it.