With security = user, you are telling samba to act as workgroup.
To let samba act as PDC or BDC, you need security = domain. In addition, you need additional settings to let your samba fully functional as a NT domain.
Domain and workgroup are two different concepts. There is a very lengthy explanation in Samba's official document to describe how to make you Samba to participate as domain controller or joining existing domains.
To decide whether you should use a NT domain depends on your scale of your current network. In also depends how much reliance on the M$ crap stuff such as LDAP.
If your scale of network is not big, say <30 computers. Workgroup should work very fine.