Really? I picked Scarsdale as a possible suburban destination. Yes, there are 15 daily trains during the "AM Peak" period into Grand Central. But there are 8 daily trains going out. (http://mta.info/mnr/html/planning/schedules/pdf/harMF_408.pdf)
From Yonkers, 15 trains come in, and 10 trains go out each weekday morning. (http://mta.info/mnr/html/planning/schedules/pdf/hudMF_408.pdf)
There are generally half to two-thirds of the number of trains available for reverse commuting on the New Haven line as well as the NJ Transit and LIRR trains. (Surely one reason for the return trips is to get trains back to their starting points later in the rush hour, but there is demand for these services or these trips would all be expresses otherwise.)
While NYC commuter rail is certainly timed to serve the bulk of those who need it, it hardly ignores those who are deciding to drive anyway.
--tim francis-wright