According to the last notification from Microsoft, they have fixed the problem where the 6th location (%ALLUSERSDOCUMENTS%\Microsoft\MSEnvShared\AddIns) that was introduced by VS 2008 when searching for .AddIn files was broken (it’s about time):

%ALLUSERSDOCUMENTS%\Microsoft\MSEnvShared\AddIns folder is not searched for add-ins
https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=466741

The fix is quite irrelevant currently because VS 2005 doesn’t support that folder and VS 2008 is broken (even with SP1), but in a few years when your add-in has only to support VS 2010 and above versions, that location will be the preferred choice because being an “all users” location:

1) It doesn’t suffer the localization problem of %ALLUSERSPROFILE%\Application Data\Microsoft\MSEnvShared\AddIns

2) If doesn’t force you to use separate .AddIn files for each VS version as happens with %VSCOMMONAPPDATA%\AddIns

See: INFO: Default .AddIn file locations for Visual Studio add-ins