It's a pointless disruption that's too difficult to be useful. It takes your attention away from other important considerations.
More often than not it's obvious when one of your opponents is about to do something. You can usually tell what players have based on the trades they offer and their reactions to dice rolls. I find that observing your opponents yields far more valuable information than attempting to keep track of who has what cards.
I make an exception when there is a single rare resource and its at a stage in the game where it might allow someone to win: For example, a player may have 8 points and 2 army cards, and needs 1 ore to buy what could be a third army card.
With that said, I do count development cards. It's also easy to tell what other players have. There are five victory point cards, for example, and when a player has been holding on to one for much of the game it is usually a victory point card. You can find a lot of information using deduction: Given the development cards you have, ones that have been played, and how many remain, you can make high fidelity guesses and also estimate the chance of you pulling out a card you need.
It happened to me recently when 2 development cards remained in the game and I knew that they were both victory-point cards, which allowed me to win the game.