I hand mine in although I have twice almost taken it home with me when food became more important than handing the card in
The BFTA grading if you score poorly for a reason can be ignored for that shoot.
IF SIGNED IN AT A SHOOT, THEN A SCORE CARD MUST BE HANDED IN.
IF A POOR SCORE IS OBTAINED DUE TO ILLNESS OR GUN FAILURE FOR INSTANCE, THEN DISPENSATION FOR WITHDRAWAL OF A SCORE CARD CAN BE OBTAINED. The Chief Marshall must sign the shooters card with reason of dispensation, the Regional or National Comp Sec may then elect to not include the shooters score for BFTA grading
There is nothing that penalises a shooter for non return of cards and IMO there shouldn't be. Cards can be lost easily and it had the potential to end up in a "I did hand it in" vs "well we don't have it so you can't have" argument.