The rules of Settlers of Catan say "each player keeps their resource cards hidden" but in my experience the game is much more fun if resource cards are visible:

• Much less time spent proposing trades. If I really want a brick and don't remember whether anyone has one, I might make several offers in turn ("one sheep ... two sheep ... three sheep ... three sheep and a wheat, come on!") needing to pause between each one to give people time to react. With open hands you're never proposing incompatible trades.

• Much more fluid trading. Having open hands means other players can model each other better, and so are more likely to propose trades that others will find acceptable. I need a brick for a settlement, and I can see that if you had one more wheat you could build a city, so I offer a brick for a wheat.

• More trading overall. The previous two points remove friction from trades, which means more trades happen. More trades means, effectively, richer players, which speeds the game up beyond just the time gained from not proposing incompatible or unacceptable trades.

I find open hands enough better that if I'm playing with a new group, even if I can't convince them all to play open hands people are usually ok with me having my hand visible. I find I end up doing more trades than other people because it's easier for people to trade with me, and trades help you a lot. There are disadvantages, like someone moving the robber has a better sense of what they could get if they played it on me, but I don't think it hurts me overall.

(I got into playing open-hand at Swarthmore where this was the standard way to play. Local legend had it that this was adopted to re-level the playing field after a student with a very good memory (hi Fred!) was able to mentally keep track of exactly what was in everyone's hands.)