I just thought I would add what I just experienced with Lufthansa and economy seat choice. We fly out tomorrow from Philadelphia to London via Frankfurt on Lufthansa, in economy. We purchased these tix back in August (amazingly, $820 each roundtrip, returning from Amsterdam) and did not pay extra for the seats. We have been watching the seating chart since then, and noticed that a bunch of economy seats were already marked out, but this did not seem to change much with time. Noticed on Sunday, just before they froze out seat selection for the first flight, that MORE seats were open. I was sitting here at the computer and out of curiosity clicked on "online check-in" at about 2 hours early (flight is 5:45, this shouldn't have started until 6:45 PM) and it let me in.
And here's the big thing I am reporting. There were a lot more seats available than we had been watching. We could choose from the first row of coach and most of the way back. In fact, that's what it gave us, row 22, we moved back one row since seatguru pointed out some issues with the first row behind the bulkhead. Similar results with our connector to London.
Now this might not hold exactly for all their long-haul flights, but it looks like most people are not paying for their seat assignment and that one can wait it out.