I got my very own Oyster card yesterday at Heathrow.
It's a little tap-and-go thing, like we have on the PATH train at home. You get a great discount on each trip, of course, but it's not like I'm traveling around London all that often. Right now, I'm just passing through on the way to Birmingham for a convention.
The only reason not to get an Oyster card is the five-pound deposit, but if you buy the card when you get here, you can turn it in when you leave and get your deposit back. And the compelling reason to get on this time versus relying on single-day Travelcards is that the cheap Hilton I prefer here (due to the pleasantly low Priceline bids I get on it), is on a bus that goes right to the train. And buses no longer accept cash, so if I want to ride the bus, I need a card of some sort.
And the last thing I want to do tomorrow morning when I need to get the train to Birmingham is figure out how to get to the tube from my hotel without a card. So fine, now I have a card! And I could turn it in before leaving and get my deposit and balance back, but I'll probably just keep it for next time.