Suddenly all my files have a null kMDItemDateAdded (Date Added) field
I don't think its a bug. An update in Apples mdimport schema will cause this. I checked this file /System/Library/Frameworks/CoreServices.framework/Frameworks/Metadata.framework/Resources/MetadataSchema.xsd on OSX 10.6.8 and OSX 10.9 and found kMDItemDateAdded only in 10.9.

When your mdworker will update your existing Meta Data using the new schema, it will fill kMDItemDateAdded with null, because PDF.mdimporter is not taking care of this attribute. How should the pdf know, when it is added?

If your mds is consuming lots of CPU time ou should re-index your Spotlight Database like mentioned here