I think you sort of answered this question yourself. I really don't think it hurts the UFCs ratings, going up against boxing and PacMan.
If somebody is into boxing enough to pay $70+ to see PacMan fight, they aren't going to second guess ordering the boxing event because of a free UFC event. They will pay to see the boxing match live, and tivo the UFC, or catch a re-run of it.
Second to that, if somebody is a casual fan of both sports, or is on a budget, they are more likely to tune into a free event (bonus for having a belt on the line).
I agree with this. I stream all my boxing fights because I hat the way boxing is these days but I love the fact that UFC gave us a free card it was awesome and all the main fights were great.