The "support" argument is almost always a bad one. Support for infrastructure products is never free; it's sometimes bundled with the license fee, but it's never free. Support is available in the form of paid consultants for any popular piece of open-source infrastructure. What commercial products DO have is official support, which provides some degree of consistency. Even that isn't a strong argument though; it's easy to tell good consultants from bad by asking for references.
My limited experience with "official" support channels is that they're manned by people trained to be "tech support" rather than actual experts on the subject.
Actually, I've talked with quite a few techs in Microsoft Professional Support that know what they are talking about. But you do have to pay a few hundred dollars per incident for that.
It's nice to know they are there when you're doing a hairy Exchange migration...
