The pub-id is the bit that tells them who to pay for the clicks. This is meant most kindly: if you understand code so very little, please make sure you always copy in exactly and never ever try to edit it - go back to the web interface and generate your new markup each time. Its against TOS to change it, so you shouldn't try to.
But Google keeps the money? An evil person would build a page on free hosting with no pub id, fill it with MFA content bait, and release click bots on the MFA ads that take the bait, that would be unthinkable. Oh the Evil ideas that people can come up with!
In this case, I beleive that the ads would run in test-mode, meaning that noone is charged and noone is paid. Even the impressions shouldn't be counted against advertisers to avoid unbalancing of ROI reports.