OK, so maybe not everyone, but almost all office based staff and plenty of others as well.
If you don't have them and you meet a customer and can't offer them your card, what does that say to the customer?
Does it say you are not very important in/to the company?
Does it say that customers shouldn't be able to contact you? Why would that be?
If you can't give something to a potential customer, how will they remember you?
It's not just in a sales environment that you may meet with people who you may benefit from being able to give a card too. You could meet them anywhere: pub, shops, gym, convention, playground, church, etc...
If everyone has them, there is no segregation of staff - No feelings of being 'important enough' to have them.
having a business card says you're important enough to a company, to spend a small amount of money on you, for you to be able to identify yourself as being part of that company.
They're so cheap it should be no decision at all anyway. You'll probably spend longer thinking about who should have them than the cost they are to manufacture any way.