_ldap._tcp.<domain> is also in practice Active
Directory specific, because Microsoft "stole" it for their own purpose.
That is, when a site sets up Active Directory they are supposed to point
_ldap._tcp.<their domain> at their Active Directory serveres.
So if the site has Windows and uses Active Directory for that, but
uses another server for LDAP, they have two choices: Fight Microsoft
and likely buy themselves a world of trouble with users who expect
the "normal" AD setup, or drop _ldap._tcp.<domain> for its intended
use. I can't imagine many choose the former.