This help topic has come up due to users reporting issues with missing parts of or all of their content on install of Revit 2015 to their systems.
New for Revit 2015 users, a new method to add Revit Content Libraries has arrived. A separate install exists to manage the Revit content libraries on the hard drive and server to download copy and install for users. Since many firms have gotten away from using the stock Revit Libraries due to the need to define exact requirements of dimensional and specifications of content even early in the design stages. For example stock content of Revit VAV boxes are nowhere near matching the overall dimensional sizing of real world content. So as far as placeholders, they’ll not be sufficient early in the design iterations.