I think the rain-sensing in AP2 does have potential, though. Look at the view it has outside compared to a small rain sensor. If they get it working well (they will get it working, but how well remains to be seen), it should actually be able to make much better sense of rain and windshield dirt than a small sensor:
The bigger problem is, is the focus area that it was watch for rain how large - or will it, for example, only watch for a small area on the extremeties of the fisheye if the rest of the view is simply too out of focus for rain monitoring...
Personally I do think they should have invested in a secondary rain sensor, too. It would have helped with the transition and been a second data point/added redundancy later on. They could have removed it eventually if it was no longer needed.
Thanks to
@verygreen for the image.