When I booked our flight with NW online, we picked our own seats at that time. When I did the online check in, I was able to go and look at what seats were available at THAT time, and we moved ourselves from the back of the plane to practically the front part, like from row 42 up to 17. To me, that is a PLUS to be seated closer to the front.
I don't think when you are purchasing online that they are showing every available seat, just what they are using for your particular fare. I could be wrong, but I know that in our instance with NW, they opened up a ton more seats at the time of the flight than were available to chose from when we booked. Just a small example, it was 3 of us traveling, for one leg of the trip, we had to pick middle seats, in 3 different rows, sort of in a row behind each other. When I went to online check in, we were able to move totally to the front and all be seated together.
My guess would be by processing your boarding passes online early enough, that if they allow you to change seats, you have a better chance of moving to something you want than if you wait until you get to the airport behind others thinking the same thing. Of course you can stay with the seats assigned if they suit you, but I prefer to sit as close to the front as possible, just from the standpoint of getting off in a timely manner. JMHO on all this, what works for me, may not for someone else, but I think its great to share experiences with others.
I believe SW is about the only one out there that won't let you pick a seat. Had this debate on other threads before, I just don't care for that way of doing things, but others love SW. I guess if it was the best fare going, I would use them, but I did it once and it was a nightmare with boarding etc., and that kinda made me put them at the bottom of my list for airlines to fly.