As I read the global rules, it seems that only callsign and signal report are strictly required for a valid exchange. WWFF reference number is encouraged, but a contact will remain valid for the program even if it is not exchanged:
3.12 Valid contact exchange
Valid contacts will include an exchange between the activator and the
hunter of their respective call signs, a signal report, and wherever
possible the WWFF reference number.
So what’s to stop someone from participating in a contest to rack up WWFF contacts? Is it acceptable? Ethical? It seems that the “hunters” in that case would likely not even know they were making a WWFF contact, but does that matter?