Aside from cultural norms as alluded to in an earlier answer, there are two main reasons people hand out their business card:
- So you have their contact information
- So you remember them personally
It seems like you're focusing on the first, but the second can be just as important.
This is true especially for the sales reps you encounter. They're trying to not only get you to buy their company's product, but also buy their company's product from them. Thus, the business card is so that if you think "Oh man, I could really use some widgets!" you don't contact
firstname.lastname@example.org, but you go straight to
email@example.com instead, and he gets the commission. Joe Salesman knows that personal rapport with the customer makes a sale and repeat business more likely, and the business card exchange is a step towards that.
Since you already have a database of contacts, this addresses point #1 - the card isn't necessary to get their contact information. However, you still need to address the other and demonstrate that you will remember them if you ever need to do business with them in the future (however unlikely you think it may be!). I've found an effective way to do this is to say something with a personal touch based on your conversation, to signal that were paying attention. For new contacts, it can be as simple as reiterating their name as if you're actively memorizing it:
Thanks but no need for the card, my company actually has your info already (smile). Joe Salesman, right? I'll be sure to call you up if I ever need some widgets! Great meeting you! (exit stage right)
This reassures the person that you are likely to remember meeting them in the future, without needing a card to remind you. You could also offer alternatives such as taking a photo or suggesting they send you an email instead, so they are even more assured that you have their information.
Another option you could try is to include a socially acceptable excuse placing the "blame" on you:
I always lose these things, but I know we have your info in the system if I need to look it up!
However, only try declining once and if they insist, take it! You can always dispose of it discreetly later on. This is just one of those social niceties -- you don't think everyone that you give a card to keeps it forever, right? It's still polite to accept things when offered and this is so little burden on you that it's not worth causing an awkward situation over, especially in a professional setting.
This answer is summarizing my corporate experiences and training about professional behavior, as well as anecdotes from a family member in sales. For further reading, this article and these Quora answers elaborate on some benefits of business cards, namely:
- A business card leaves a stronger and more personal impression than simply looking up someone's details
- Cards are a physical reminder of the person, so if you promise to follow up and later forget, there's a better chance you'll remember when you see the card again
which helps to explain the other person's motivations for handing one to you.