A surprisingly large number of people will receive a business card and put it in their pocket without looking at it.
With the amount of work and thought that can go into designing a business card, what impression does it give if you can't respect that while the person who most likely was involved in designing it is right in front of you?
This is something that most of us have done at some point and there are practical reason why you might want to do this, whether it's to write down a number, remember something noteworthy about the person so you can associate them with the card, or to note something else relevant down for your own reference.
Just don't do it in front of them, because when you do you are treating the thing they put so much time into designing and making presentable like a piece of scrap paper.
If you really must write something down while you are talking to a contact, then carry a notebook and ask if they would mind noting something down (99.9% of the time there shouldn't be any objections).
There will be occasions you will wish to do this, but do it discretely since it can leave a bad impression not just with the person who gave it to you but also with other people potential clients you might may contact.
Doing it when you have just received it is also plain rude.