It's probably not useful to add the crosswalk unless the sidewalks are also added.  Otherwise you can just create a new node on the road here and tag it with highway=crossing and crossing=marked.  That being said, explicitly drawn sidewalks are not totally 'redundant' (and I agree it's somewhat of a waste of memory), however, it's not wrong and people do frequently add them, there are places with an extensive sidewalk map with crossings.  Up to you how you want to proceed.