Basically it's for my parents, who always fly LH long-haul via SIN. Their standard procedure is to purchase C tickets though I have noticed in my dad's M&M account that he has 8 eVouchers. So this time I thought I might just get Y tickets and burn those vouchers to get rid of them.
F is a nice step-up but as they're on a night flight they're just looking to get some sleep (I think the FRA-SIN flight takes off at around 22:00) and have always found C to be perfectly fine.
They did get upgraded from C to F once. Upon boarding they discovered that there were other people in their sears so they asked the FA for assistance. It turned out that the people in their seats and my parents had BPs with the exact same seat numbers... hmmm... interesting! I can vouch for this story as it happened to my parents and not a friend of a friend of a friend.
Anyway, C was full but the FA acted professionally and moved them upstairs info F.
Moral of the story... don't be the first to get onboard! You might just get lucky and get bumped into a higher class of travel (though things would have been messy had F been full or only had one free seat available...).