I feel I should counter this unlucky story with my my brother's own story of seat assignment service.

They have no status and were flying on saver awards from LHR to SFO and had 32AB and 33AB as their seat assignments. On check-in at Heathrow the check-in agent commented that they weren't sitting together which with two young children didn't seem right. Without them even asking she called for a supervisor who apologised profusely, but explained that they were now in Economy plus. Eventually he got them 21CD and 23CD - no window but extra leg room and not too far apart.

He never explained why they lost their original seats, but the quality of service and their proactiveness in seeing and trying to fix the problem really impressed my brother.