Same things were said when the asicminers old USB miners were being sold. And it was probably true. Didn't stop them from selling like crazy.
By the time gen 4 USB sticks could reasonably be expected to ship, Id WAG difficulty to be above 80B.
2.5W/0.3J/GH= ~8GH wich would yield 0.0000501 BTC per day.
Last summer, a USB block erupter mined about two orders of magnitude more.