The directory server updates the modifyTimestamp on operations other than Modify operations. See http://tools.ietf.org/html/rfc4512. However, attributes can be modified by operations other than Modify. For example, with password policy enabled, a Bind operation may update one or more operational attributes in a user's entry. Admin's do not expect modifyTimestamp to be updated under this and similar circumstances. See also http://www.openldap.org/its/index.cgi/Software%20Bugs?id=5040;page=2
This falls under the category of RFC correctness
See also bug 453756 and bug 305131
We need this for openldap parity
Upstream ticket: https://fedorahosted.org/389/ticket/129
This was fixed in 389-ds-base-1.2.11-0.1.a1.fc17. Closing.