I am trying to gather some statistics regarding the difference in NA27's use of the early versions. According to my electronic search, the 'rarely cited' versions produced the following number of hits from the apparatus criticus:
OC Slavonic 0
This is a steep difference, when one considers that the Coptic versions are cited 1057 times in the Gospel according to John (the above numbers are for the whole NT). I understand the reasoning behind this, but am still somewhat surprised at the contrast.