I did not bother doing on-line check-in as we had a fair amount of luggage and the Lufthansa check in counter and bag drop are the same counter in Manchester.
But at least you would have a valid argument if you did check in on line.
But no-one would have to be shifted around if Lufthansa had blocked off five seats right when we booked the tickets.
But again, you had the opportunity to block off 5 seats yourself - but you 'did not bother'.
I am not disputing that airlines can and do amend pre-booked seating, but by checking in on line does give you an increased chance of getting the better available seats than not being bothered to do so expecting the airline will seat you together by default.