I was told by a fireman/neighbor:
1) In adults, the bloodstream carries sufficient oxygen for the task (until pro help arrives) and that the big problem is getting enough flow through the heart to get the oxygenated blood to the brain. Stopping pumping to give a breath is more detrimental so it's best to keep pumping.
2) Some people might not wish to give CPR if they have to breath into another person (touch their mouth). Removing that component might have more people offering to assist.
Now, if a second person is there to give a quick, deep, breath - I guess it'd be good - so long as pumping is good.
