I fly pretty regularly. Sometimes on SW, sometimes other airlines. The open seating thing doesn't really bother me.
The last 15 years we have had family boarding when flying with family and I have A-list Preferred when I'm traveling by myself. So our seats are never bad. I can see that you might be bugged if you get stuck in Board Group C and got some of the worst seats on the plane. But if you care where you sit, is the $15 for Early Bird Check-in a big deal?