Me Me Me I want more :p
I think having the ability to have more pins is not essential by any means and ReefPi is amazing with what it can do already, but more pins would certainly help with larger tanks (120G+). Not with the power side of things, 16 outlets really is all I can see myself controlling on my 180G and if I need more than that I can always just use a normal power board for equipment that is always on and doesn't need to be controlled.
But more pins is great for the monitoring and subsequently controlling equipment side of things. When you think about how many places you can put float switches, flow meters, leak detectors etc. on a larger tank, all to let you see the health of your tank from a glance / give alerts when things go wrong the number of pins can add up really fast (large tanks that run on commercial controllers usually have at least a couple of break out boxes full of float switches etc).
For example if you have a tank with an external skimmer, algae scrubber and a UV sterilizer and want to monitor the flow rates through all 3 pieces of equipment that is 3 pins right there. Then I can name at least half a dozen places I can use a float switch to monitor / control my system so the pins really add up fast.
But having said that none of it is essential by any means so it's not a super important feature.