Foam is the best and safest way to clean finished leather. The foam should be dense enough not to run on surfaces which is the problem that we have with liquid detergent cleaners that can etch into the finish if left to run down the leather.
You are only dealing with the finish coating on the leather (unless you are cleaning unfinished leather which needs a wholly different approach) so you can give a foam real dwell time - the product should do the work without the need to scrub the dirt out - if the foam does not lift the dirt out then it is either not a cleaning problem or you are not using the right foam!!
You can agitate the foam with a brush but this is not scrubbing the leather - a foam will encapsulate the dirt and enable it to be removed fully from the surface otherwise you are just moving the dirt around
Hope this helps
Judyb