Sharing a bib might not seem like a big deal. But what if it's done to qualify for Boston, NYC or Olympic trials? Someone honest gets robbed of their rightfully earned chance. What if that's you or your friend?
What if it's done to set a (local, state) record in the wrong age group or gender and it's your record that's erased?
What if it bumps you out of an age group or masters award? You might not care, but some people would and that's their right.
Add to that the fact that there's no real reason anyone needs to borrow a bib.
Sign up yourself. Is it that hard?