My dentist consistently knows whether I've been brushing much or not, and when he does a cleaning it hurts a lot more if I haven't been doing it much. Also, after four or five days of not brushing my gums start to hurt, and they feel a lot better after brushing. That of course is consistent with e.g. the fact that you start to itch if you don't wash other parts of your body and so on. So that seems like good evidence that brushing is at least as useful as washing in general, even if it is only anecdotal insofar as that is my personal experience.