Currently, I can capture traders, never buy anything from the port, never visit another port (aside from capital) and build anyship i want. What is my motivation to capture a port?
At the moment I think they're just cosmetic. Unless I as an individual gain a benefit I see no pragmatic reason to invest the gold to capture one.
It really doesn't matter what the benefit is, cheaper prices/repairs/kits/tax as long as it's something tangible.
There's lots of discussion about flag timers, conquest windows and even port production/buildings but there is currently no discussion as to actual benefit. I was super excited to claim the entire Caribbean for king and country, until I realized that it didn't matter at all.