While (as you rightly pointed out) it is quite clear there are advantages for and against individual opensource an proprietry products, there is also an argument to be made for opensource in general.
This is not to say that every open source product has better (or even equivilent in some cases) functionality, but that the very fact that it is open source has benefits. For a large multinational such as the submitter is enquiring for, one of the big wories must be ownership and continuity of support for whatever product / projects they use in their IT infrastructure.
Pick a proprietry product, and a company going bust or mearly becoming uncooperative could result in a large risk to your ability to maintain your internal infrastructure - be it through bug fixes or introducing new features.
By choosing an opensource strategy, it will always be possible to either maintain such systems internally, or shop around for someone appropriately qualified to make the changes you need. Purchase and maintainance TCO are good arguments, but IMHO the biggest factor to large multinationals will be one of reduced risk, and therefore there can be a benefit by choosing a lower featured opensource product over a traditional proprietry one.
