Seems easier to simply purchase bitcoin than to spend time on those sites.
While this is true for most people; you forget the kids who want to get into BTC. They don't really have any means to purchase the coins and start an interest in them.
I also recently posted this elsewhere today, which explains why some think free bitcoins sites have a role to play, even if they give away very little:
What good is it if one only gets very very small amounts?
https://bitcointalk.org/index.php?topic=74046.msg1088930#msg1088930