I believe it's actually possible to get from Boston to Washington D.C. via public transportation (with a little bit on Amtrak between Providence and western Connecticut), so yeah, it reaches much further than 10 miles. ;-)

The problem is time. I live in Bedford, and while it's technically possible to get a bus+subway from here to the financial district, it takes about an hour and a half. That's compared to 35 minutes driving (no traffic), ~55 minutes driving (with traffic), an hour driving to Alewife then taking the red line in, and an hour and a half for biking to Alewife and taking the subway in. Not much point.

Commuter rail is similar - it's like 1.5-2 hours to take the commuter rail from Concord to Porter and then transfer to the red line. Might be a bit better closer in to the city, but commuter rail coverage is a bit spotty - it's faster to drive to Alewife from Bedford than it is to get to the nearest commuter rail stations in Concord or Lincoln.