You've obviously never tried to hail an Uber from a dodgy neighborhood in Boston then. It takes me twice as long to get picked up where I live, versus my girlfriend, and I generally have to go through a driver switch or two.

Well yeah it takes a little longer because drivers aren't going to hang out in that neighborhood. You do eventually get picked up though. My experience in Baltimore with taxis is that no amount of calling will get them to come to a part of town they don't like. Nothing is going to change the fact that it is a crappy part of town but the difference between no pick-up ever and pick-up with a small glitch is huge for a lot of people living around those areas.

Is a Uber or Lyft driver required to accept any pick-up?

No, but, they track acceptance % by drivers so a driver that is screening locations like that will put themselves at risk of being kicked off Uber. The expectation is that the acceptance % is very high and that once a fare is accepted it is 100% carried out. Also when they display a potential fare to a driver they only show an estimate of how far away it is in minutes and a very zoomed in map of the location so it is hard to even know where you are going to be headed exactly before you accept.