Do YOU think we'll get a new Tank and/or Healer in 4.0? And vice versa.

[ChaozK]

with 3 jobs there are 3 non-duplicating combinations for 2 party spots. all the jobs are viable for 66% of the combinations.
with 4 jobs there are 6 non-duplicating combinations for 2 party spots. in your example BLM and SMN are viable for 50% of these combinations, BRD and MCH are only viable for 33% of these combinations.

viable 33% of the time is not the same degree as being viable 66%. 66% is literally twice as good.

Nah its really not like that. You are assuming that all those comps have equal weight but in reality doublecaster comps are suboptimal. As long as BRD and MCH are part of the optimal raid comp it really doesnt hurt BRD and MCH players if suboptimal comps get less suboptimal.