The wikipedia page for CPR actually goes into some detail on this, going to block quote the section here for ease of access,
Compression-only (hands-only or cardiocerebral resuscitation) CPR is a technique that involves chest compressions without artificial respiration. It is recommended as the method of choice for the untrained rescuer or those who are not proficient because it is easier to perform and instructions are easier to give over a phone. In adults with out-of-hospital cardiac arrest, compression-only CPR by the lay public has a higher success rate than standard CPR. The exceptions are cases of drownings, drug overdose and arrest in children. Children who receive compression-only CPR have the same outcomes as those having received no CPR. The method of delivering chest compressions remains the same, as does the rate (at least 100 per minute). It is hoped that the use of compression-only delivery will increase the chances of the lay public delivering CPR. As per the American Heart Association, the beat of the Bee Gees song "Stayin' Alive" provides an ideal rhythm in terms of beats per minute to use for hands-only CPR. One can also hum Queen's "Another One Bites The Dust", which is exactly 100 beats-per-minute and contains a memorable repeating drum pattern. For those with non cardiac arrest and people less than 20 years of age, standard CPR is superior to compression-only CPR.
The American Heart Association has assembled some efficacy testing studies in the following review article, Hands-Only (Compression-Only) Cardiopulmonary Resuscitation: A Call to Action for Bystander Response to Adults Who Experience Out-of-Hospital Sudden Cardiac Arrest. Importantly, the human clinical studies reported no significant difference between those who received conventional cpr and those who received compression-only CPR. Some animal efficacy studies are noted, and with respect conventional CPR, the animals that didn't receive "rescue breaths" experienced a lower survival rate. In the end they note some limitations to hands-only CPR:
"...raised concerns about the possibility that recommending hands-only CPR for witnessed sudden cardiac arrest will, in fact, increase the complexity of decision-making for bystanders or that unresponsive victims of noncardiac medical emergencies (eg, drowning, drug overdose) will not receive rescue breathing."
"There may be situations in which ventilation alone could be life-saving but is not provided. There may be an interval after cardiac arrest when ventilations become absolutely critical for survival."
And as to the content in the second paragraph of the original post,
"It is important to acknowledge that during cardiac arrest without lung inflation and ventilation, there is a continuous decrement of blood oxygen saturation. At some point in time, the possible hemodynamic advantage conferred by continuous chest compressions (without ventilations) will be offset by this reduction in oxygen saturation, and the ultimate result will be a compromise in oxygen delivery. One porcine cardiac arrest study (3 minutes of untreated ventricular fibrillation, then 12 minutes of CPR) suggests that after 4 minutes of continuous chest compressions without rescue breathing, the delivery of 2 rescue breaths every 100 compressions provides a survival advantage over chest compressions alone."
The paradigm, again, is that hand-only CPR provides opportunity to get some help, other than no help at all, but the review also notes that more clinical data is needed on all fronts.