Great idea, but limited...
If the upper spend limit is under £10 to prevent fraud, then I challenge ANYONE to find useable parking in central London for that amount or under...OK, you can, but it's very close.
But it would be very useful to have such a swipe card for buying the paper, coffee, etc. I would expect to see these card readers in all of the shops adjoining Tube stations.
But overall, I think that Oyster is one of the best things about London transport, and it's sheer efficiency and speed almost makes up for the crammed, un-airconditioned trains that pass as the Tube...or those damned, traffic jamming, public-injuring flexi-buses.