The reason the other guard did not want him giving you the ticket is that technically, he's not supposed to! You are supposed to only get the Charlie Card at ticket windows. I believe, but am not certain, they are supposed to see a license. Think of it as a monthly pass. They give a lower rate to get more people to use it (and also track your ridership data!). Tickets are kind of the walk-up price.
Most people buy cards if they can get to a counter. If you stop into the city once every couple of months and take the T twice the savings isn't worth having to go way out of your way for it. If you are planning on visiting for a few days, though, make the trip somewhere to get one and save the pennies.