Very interesting.. I used before the Canon S80 with a sliding lens cover and I find myself having the tend ency to touch the lens accidentally especially when it gets fully retracted and the cover not closed.
Unlike the S80, the lens of the LX-2 (or LX-1) still has a protrusion which actually prevents your finger from accedentally touching it when it is retracted. Initially, I thought the protrusion might be cumbersome, but on hindsight, I think it does serve a purpose.
So I guess a filter is not really necessary but good to have if some care is put in when handling the camera.