It's literally one e-mail and one additional mouse click every 3 months to activate a category. They send the reminder e-mail stating the categories, I click on the Activate button in the e-mail, boom. Done. Most I think also have their calendar of categories posted online so you can see what's coming up.
To each their own though.
As to the other debate, I rarely use my debit card anymore. I could spend, say $1000, a month on my debit card and not get anything in return. Or I could spend that $500 on a credit card and receive at minimum $10 back. Not a lot at first, but adds up over a year especially with 2, 3, or 5% back categories. I'm single, no kids, live in a small apt and don't have a lot of expenses (and am trying to save; a little kickback here and there helps). I imagine a family of four easily brings in a substantial amount in cash back every year.