For the sake of clarity regarding a possible rain on the planet Mars, I Toggle some pictures from one of the security cameras the Blue Lagoon uses. As you can see, it rained that day. The reason why the protecting glass of the camera is not stick to the camera lens is that it would affects the focus of the camera. Another reason is the airflow, called ventilation.
No, it isnt similar. 10cm is NOT next to the window. Try again with the lens of the camera right up touching the window and see if you get any outline of the raindrop. This is likely what we have on the rover as the lens is not likely to be 10cm back from the saphire (on MER, anyway) window of the camera.
I am not saying that water will not 'stick' to the window, although they probably have coated the lens window with anti-static and other chemicals to stop the rain/dust/particles sticking to it so the camera can still get images even in dirty conditions on Mars.
Find a piece of dirty window and do the same for dust and dirt and I bet you will not be able to see the individual dirt/dust spots in the image.
Out of my window, rain the picture taken today 08-30-2012. The distance between the lens and the glass is 10 cm.
Thickness (mass) of water droplets on the glass depends on the resistance of the glass and the effects of wind also gravity of the planet. One drop on the image corresponds to what is on the picture from March, marked by an arrow.
I have no doubt that it does rain and that would mean that raindrops would get on the lens of the Rover.
BUT... (although you may not agree with me) ...
Have you considered that it is almost impossible to see anything which is ON the lens of the camera? Anything on the lens of the camera shows as a (slightly) darker smudge or out-of-focus area and does not appear in the same 'plane' as the image in the photo. Dust on the dirty lens of my digital camera appears like this. If we had the camer lens.... then a foot or two and then the lens cover, you may see blobs appearing as raindrops like you do on your car windscreen, but I highly doubt that this is rain showing
If we go further into this picture, we see the rain approaching the Rover. We see directly ahead, the sun shining through the mist(Reflective sunlight on the Rover) and the first drops of rain began to fall on the glass casing the camera.