Are there benefits to opening a flight plan if I'm flying over populated areas?
According to the AIM section 5-1-4, the main reason is search and rescue:


  b. It is strongly recommended that a flight plan (for a VFR flight) be
  filed with an FAA FSS. This will ensure that you receive VFR Search
  and Rescue Protection.


So the question becomes, does search and rescue have value in populated areas? I would say that - excluding large cities - populated areas are actually not very densely populated at all (relatively speaking), and there is a very good chance that no one will see your aircraft come down or even react if they do. You might be flying at night, or no one may be looking in the right direction, or they may assume you're landing at a private strip or doing cropdusting or whatever.

And even when an aircraft is known to be down in a specific area, it's often very hard to find it - especially if you land in trees - so anything you can do to narrow down the search area is important. An AOPA safety video (I forget which one) mentioned an incident where a helicopter went down less than a mile from a runway threshold at a class B airport and it still took 24 hours to locate it (the details may be different, but you get the idea).