Southwest doesn't have seat assignments, so no, they don't charge for them. If you have one of the lower fares you can pay for a higher priority boarding number, but that doesn't guarantee you a specific seat.
I think PP was saying that if you miss your flight, Spirit does not usually allow you to fly standby for free in another available flight. I fly between LAX band Vegas a few times per year on Southwest, as they have flights almost hourly, and I've met a few people who missed their flight because the gate closed earlier than they were expecting. The woman I sat next to this past May has shocked that I checked my bag because it was free, whereas she was charged for a carryon, and then denied boarding. But if you know what you're getting into with them, and you take into account the fees, they might be the best deal.