250.50 Grounding Electrode System.
If available on the premises at each building or structure served, each item in 250.52(A)(1) through (A)(6) shall be bonded together to form the grounding electrode system. Where none of these electrodes are available, one or more of the electrodes specified in 250.52(A)(4) through (A)(7) shall be installed and used.
It doesn't say "shall
be permitted to be bonded", it says it "shall be bonded." Period. By the NEC, all the available grounding electrodes shall be used.
See 90.5 for a refresher on
mandatory versus
permissive language.