I disagree with you there. Yes, the players should know that there is a threat pool, but knowing it is there and seeing exactly how much is there leads to (in my opinion) players actively not taking risks that would be great for RP/gaming because they know there won’t be any negative effects or repercussions.
Those situations, and the meta gaming of knowing how much threat is in play can lead to stifled games. Granted, that might not be the case with every group, but neither is the ‘visible threat in the middle of the table’ way of dealing with it. It should be handled however your group would like it.
Another possible scenario of the threat in the middle of the table is that players won’t actively add to the threat pool because they can see how much is there, and might not want to give the GM more ‘ammo’ as it were if he is low.
As I said, I think it should be based on your group. If you have had a similar mechanic before it worked fine with everyone seeing it, then go with that. If you haven’t had this type of thing in play before, maybe for the starting game have it sitting visible for them to see for the first session, then keep it hidden the next session (while still letting them know you are spending it). See which method provides the most fulfilling RP for the entire group, then go with that.