The one valid situation that I can imagine that would justify soft breaking must satisfy four criteria...
1) For some reason you cannot make a ball on the break at that table after repeated tries.
2) Your opponent has no problem making a ball on the break.
3) Your opponent can easily run out an open table.
4) You are a better mover than your opponent.
If any of the four criteria aren't met, you might as well smash the rack since a safety break would not help your situation.
In Vegas about 6/7 years ago. I was playing this young guy, maybe 18/19 years old, who made every open shot he could see. My break wasn't working and he was running out like water when given the opportunity.
So, looking above to your criteria....#1, #3 and #4(in my mind) all came to play.
I managed to tie the score at 4-4 and it was my break for the final game. I decided to do the 2nd ball break and played the speed to come off the side rail and back into the stack. I probably hit the break at 12/13 MPH. It was a legal break with enough balls hitting the rails and the cue ball nestled into the stack perfectly. He had no shot from there.
His Uncle was watching the match along with a close friend of mine. The kid turned to his Uncle and said WTF was that? He studied the rack for a bit and went for a flyer, missing for almost the first time in the match. I made a couple balls and played another lock up safety, giving him absolutely nothing to shoot at. During that inning I managed to knock a couple more of my balls into the open before the safety. The kids body language showed anger and complete disgust for the whole game I was playing.
He attempted a kick, giving me the table back. I proceeded to win that game and the match 5-4. Afterward he quickly shook my hand but started talking to his Uncle about the BS style of play. His Uncle told em "it was all legal and within the rules, you just need to learn more about 8ball". It was priceless when his Uncle shook my hand and said "good match".
Anyhoo, sometimes you need to adjust a bit in order to pull out the win. It's not always about smashing them hard and running out.