To often we have to recreate a card in multiple dashboards (that serve different purposes) and it is easy for these cards to get out of sync with each other (and a lot of work to keep them in sync). I'm sure the when any card is created it is given a unique ID in your database to store the configuration settings and therefore we could reference this number to add a card to multiple dashboards just like we can add a task to multiple Lists. Giving us the ability to add this card to multiple dashboards would provide us the ability to ensure the same data no matter which dashboard is being used.