What do you think?

[WVREF]

You pointed out one of the reasons to rule whether pass was forward or backwards even if complete. If backwards no roughing the passer. Another reason, but not very common, would be if this were a double pass play. If first pass was forward pass interference restrictions on B would be off on the second pass. If first pass was backwards then B's restrictions would still apply. Good luck explaining the difference to the coaches.