definetely to it, independent of their debt situation.
i honestly don't think of cash as a liquid monetary source anymore! with cash i can't pay my bills, order anything online, buy anything expensive (without a special trip to the bank, anyway). i have little consumer protection with cash, and it's dangerous to carry a lot around with you. i still carry a little for toll booths and super small purchases (i.e, gum, pencils, etc). but if it costs more than $5, i use a card. and checks are super slow, cost money, and offer no advantages over plastic.
with plastic, i get a free loan on all my purchases for a month, then write one check (or just transfer money online) to pay for everything. in the meantime, i'm earning interest on my money in my bank account (not that this adds up to much, but still). if it gets stolen, i just report it and i don't lose any money. and using it regularly helps build credit history. some cards offer further benefits, though mine are pretty much straight-shooters.
some people get into trouble because they can't keep track of their purchases or they buy things they can't afford, but a fool and his money are soon parted regardless. last i heard, americans are more steeped in debt than ever.
the downside? increased costs! retailers raise the prices of their merchandise to cover credit card fees, so we end up paying for the priviledge of using the card. but to me, it's worth it.