Agree that seats are not guaranteed, nor is the flight for that matter, but when facing 12hrs+ down the back, seat selection or lack thereof can make all the difference to the experience, and I admit that the further towards the pointy end you go the less the actual assigned seat really matters as most 2 2 2 or 1 2 1 configs will guarantee you that most essential item - personal space - unless if course you are flying BA with their 2 4 2 config in J.
So, before pushing the Pay button I want to be sure that I have done everything in my control to try and get the most space / comfort available to me. Before the advent of the i-net I would be happy to go with the status quo and accept my fate at check in . Middle of the middle at the back allocated.... Doh........ . Note to self - check in at airport earlier! But these days I prefer to at least try and reduce the odds of such an allocation by looking at the seats available prior to payment and making an informed choice; albeit knowing that I may, just may get moved. To date that hasnt happened. And being the shape of Sponge Bob square pants I will continue to avoid the 'pot luck' strategy in an attempt to select and keep aisle/window