While they do provide one of the leanest implementations of an ActiveX control, they only have a minimal number of interfaces implemented. And those interfaces are only the ones that are needed to function properly inside of a Web browser. This means ATL Lite controls will fail in containers like Word, Excel, and Test Container, which require more interfaces for proper operation.
Reference:
PRB: ATL Control Properties Do Not Persist when Embedded in Word
http://support.microsoft.com/kb/q241936/
Microsoft Office Development with Visual Studio
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnoxpta/html/vsofficedev.asp