Continuing, sometimes marketing gets out in front of a product's capabilities, hurting both the IT buyer and the IT vendor. In other words, the CIO ends up with a product she didn't intend to buy, and the vendor ends up with an unhappy customer that it didn't intend to attract.
As a case in point, CMS Watch has been tracking Microsoft SharePoint uptake and discovers that despite early hype from Microsoft's channel partners, SharePoint isn't or shouldn't be all things to all people. Because of Microsoft's product strategy, it has helped customers reach this realization with minimal pain:
Almost everyone licenses SharePoint at some level within their organization, almost everyone already uses legacy ECM systems. Almost no-one is planning replace these legacy systems with SharePoint.
Discussions with integrators and channel partners have also been consistent. The marked enthusiasm of the channel just a year back for SharePoint as a "Documentum or FileNet killer" has gone. In its place is the dawning reality that SharePoint is exceptionally good at a one thing -- group and department level document collaboration. It's not that it can't perform other services, it can - but they are not its sweet spot.
To Microsoft's credit, it offers SharePoint Services for free, which allows potential SharePoint Portal (i.e., paid) customers to evaluate the product for their use case before purchasing. Otherwise, some might discover that using SharePoint for web content management, for example, is not necessarily a great idea.
In a similar but more profound way, open source cuts through marketing hype to lay everything bare before the buyer. Can Open-source Product X handle my Business Intelligence requirements? Download it and try it out. Could it be tailored to fit my enterprise's needs? Muck about in the code and documentation and see for yourself.
Open source, then, is the ultimate weapon against false hopes created by false or merely hopeful advertising. Even if an open-source vendor were inclined to lie about its product capabilities, the code never lies.
As Microsoft demonstrates, open source isn't the only way to mitigate IT purchasing risk, but it is the most thorough and conclusive method that I've found to do so.
Disclosure: I work for Alfresco, a company that offers a competitive open-source product to Microsoft SharePoint.