Actually added in version 13:

Changed the C-mode/D-mode selection rules in Section 4.3.3 to make the use of
C-mode a SHOULD unless capacity can be explicitly engineered.  Also added a
reference to this fact in the rate control section, Section 5.3.3.

In 4.3.3:

   o  congestion control: D-mode SHOULD NOT be used for signalling where
      it is possible to set up routing state and use C-mode, unless the
      network can be engineered to guarantee capacity for D-mode traffic
      within the rate control limits imposed by GIST (see
      Section 5.3.3).

(it used to just be a 'MAY' to benefit from congestion control)

At the end of 5.3.3:

                                                           ...  Note
   that, according to the rules of Section 4.3.3, in general D-mode
   SHOULD only be used for Queries and Responses rather than normal
   signalling traffic.