I've been recently involved with a software procurement process /rfp recently.

The manager in charge of the process was A LOT more interested in CYA and selecting the product that had the most documentation to defend his choice (Gartner quadrants, other customers, etc.) than in taking risks or actually selecting the "best" product or sticking his neck for whatever he believed the best provider was.

Not unsurprisingly, Oracle and IBM provide a lot of what he needs to show upper management, and so tend to be in the discussion a lot.

You don't have to defend hiring IBM, but you DO have to defend using open source software X with support from local company Y. RedHat and similar do provide a level of "IBM-like" services for large companies, but most open source software doesn't have the support levels and on-site teams required (or sometimes IBM offers those !! Edit: as someone else pointed out, Oracle offers MySQL support too).

smtddr's point about having someone to phone is VERY true for these kind of conservative companies. (also, I've seen the kind of behaviour he documents about people trying to - and succeeding at - claiming credit for projects)

To be fair I think there's a certain amount of momentum and economy of scale when you reach enterprise levels of anything. Incrementally it's quite cheap to toss one more of widget X on the pile when you have a room full of widget X experts.

If you decide widget Y is now the way to go, you get to answer all the questions already answered for widget X: Who supports it? How? How to do we back it up and restore it? Who can tune queries? How does this differ from our build patterns for servers that host X? Can the two live on the same machine or are there potential conflicts? Etc.

Don't get me wrong - with proper planning you can educate the X guys on how to support Y and smoothly add it or even transition to it. But for large-scale systems it requires a great deal of planning and forethought if you want to do it without any bobbles.