The cheapest RFID chips - by and large - are not read/write. They're read-only. The Wal-Marts of the world aren't putting read/write RFID in their products. This strikes me as largely a non-issue.
As far as the securty-badge scenario; you'd have to be pretty close to the badge to get it to transmit. Like, close enough to have it in your hand. If the bad guy has your badge in his hand, you've already got bigger problems.