With respect to small plane airlines, there's no benefit to pursuing a higher CI nor can they afford to based on the small nature of their operations. If you're running all domestic flights with little to no business demand, it doesn't benefit you to go very high on it.
Having toyed a lot with SW3 in last GW3, I think it's more complicated than that. Very small planes usually come in the most economic seating. And there is a hard limit to the pricing you can set. On some lines, you've got a 95%LF at price 180, and 65% at price 190. CI raises this bar up - and it's really a much-needed oxygen.
But when you're small, you can't get it easily, and marketing is prohibitive. Which is realistic. So you stick with lower prices - or switch to standard seating, but it's not without drawbacks either. I made a comparison for all my SW3 flying from Fuerteventura(15 of them), and at the end, in terms of profit, standard seating and its 16 places instead of 19 was not better, despite much better pricing.
So CI is good, even for small airlines, but they simply cannot afford it, usually. When I began to have very juicy E195 routes to pay the raise of my CI up to 90, though, I could lift the pricing on my SW3 much higher. But still with a hard limit, above which LF falls brutally.