Conversely, the choose a seat option may make the plane look fuller than it really is. Many airlines block off sections of the plane (for example the first certain number of rows are blocked for frequent flyers, the back few rows may be blocked for flight crew if the flight isn't full). If you aren't a frequent flyer with that airline, it would look like every seat in the first bunch of rows is taken.
Further - that option doesn't help at all for SW since there is no "choose a seat" option. It helps only slightly more for airlines like Spirit/Air Tran where you have to pay to choose your seat, since you have no idea how many people have booked a ticket but not paid to pre-choose their seat.