When an ActiveX object is insertable and a control then it will not appear in Microsoft Office 2007 and later, in addition it will not work with Office 2003 but it does appear as an object that can be inserted.

The reason for this is that the insertable and control entries are both in the registry which causes problems for Microsoft Office. However, it would appear (according to http://msdn.microsoft.com/en-us/library/aa751972(v=vs.85).aspx#Self_Registration) that the control entry is for old hosts and there is another way that registration as a control should be done so that it avoids this conflict. This should be investigated further.

Update: After some communication with Microsoft, it seems that the registration as a control has to stay because it is Powerpoint which seems to be the main issue here. However it is still a valid thing to do to investigate the other documented approach (with the control categories) further.

More update: It seems that with 2010 it will not work at all even if it is just marked as a control