On every auto with a hammer that I know of (and that's a few), cocking the hammer first does reduce the force needed to pull back the slide. For some people, that can mean the difference between being able to cycle the slide, or not.
Yes, pulling the hammer all the way back and letting it drop onto the sear is a bad thing. But how many people are actually going to do that, especially once they understand how it is a bad thing? Most will just pull back until it "clicks" at full cock, or (as I do) pull past the full cock click, then ease it down until held.
Yes, it could slip, and that's a bad thing, but anytime a hammer slips, its a bad thing, now isn't it?
All else being equal (and it almost never is) bigger bullets tend to work better.