I'm an EMT-Intermediate, I work part time for a 911-only EMS system and I have done CPR for real. I have been the 2nd man on many ACLS calls (drugs, manual defib, 12-lead EKG, etc). Some points:

1) the little fold-up shields are definitely better than nothing, but if you're doing rescue breathing on your own (no BVM) I prefer the Laerdal pocketmask (or equivalent) with the 1-way valve. I keep a foldup shield on my person and a pocketmask in my bags/kits.

2) the AHA implemented compression-only CPR not because no one ever needs breaths but because untrained bystanders were avoiding doing any resuscitation at all due to valid fears of infection. The AHA has found that uninterrupted compressions are more important than breaths, so they figured compressions alone was better than nothing. If you have any decent training, however, you should include breaths and that leads to bullet point number 3...

3) cardiac arrest is NOT the only reason you may need to provide rescue breaths. Head injury, lightning strike, near drowning and overdoses are situations where the victim might have a pulse but their respiratory drive has been short circuited. Some properly-applied rescue breathing can potentially save their life.

"Coming back from the dead" after cardiac arrest is rare. Coming back after respiratory arrest is not as rare. It would be a shame if you couldn't provide those critical breaths.

Just as it is with so many of the things we discuss here: knowledge and training are more important than equipment selection. But when it comes to equipment, get the best you can carry.