I contend that Southwest's process delivers better seats than assigned seating to the people who care most about it. People who don't care very much and are therefore not motivated to make any effort (such as T-24 checkin or paying EBCI) will do better with assigned seating. And of course, people with status who are upgraded do better on airlines offering that.
It's well documented here that checking in at T-24 or buying EBCI does not guarantee a specific boarding position or seat will be available, which is why I believe that Open seating means a broader range of outcomes, even for people who are proactive about their BP #. A flight could have 100 Preboards, BS, A-listers, and families, or next to none. EBCI is a $15 wager that most other passengers didn't also buy it, and that no one with a worse boarding position gets to skip your place in line. On some routes, that same $15 will buy you an assigned favorable/extra legroom seat with a Legacy carrier.
That said, if you have an assigned seat, and upon boarding, find that your seatmate/s are objectionable, you'll have a harder time swapping to another seat. On Southwest, unless you are the very last person boarding, you will be able to pick the least bad remaining seat.