If your employer responds to the number of employees who are giving by modifying the cap, then that means that whether or not you give will not change how much money your employer gives in the long term.

However, even if that is true, you still are choosing where your employer donates your portion of the gift match. Therefore, if you believe that most of the money given to charity is being used at only moderate effectiveness, but that you could choose a good givewell charity to donate to and achieve much greater results, then the impact of the employee match is still signficant.