Well, what if you're under fire? You need to help em FAST.
Also... maybe regular revive, you get up with only 1/4 health, but defib gives you all?
If you are under fire you don't need to be trying to revive someone, STILL IN THE LINE OF FIRE. If you and your down teammate are under fire, return fire and kill the person, THEN revive. Otherwise you will be open to being shot while you revive them, and once they are revived they will just get shot again too.
And its not like defibs will make bullet wounds go away, but it makes more sense for defibs to revive someone rather than just helping them stand up.