I can think of very few situations where a soft power switch has provided a noticable benefit to me. On the other hand I can think of quite a few occasoins where I was seriously annoyed by them.
My last computer (ATX) had some sort of screwed up mobo or power supply. About twice a month, it wouldn't come on when you hit the soft-power button for some reason. Every time this happened, I had to crawl under my desk and yank the power cord out of the back, plug it back in, and then it would work.
Then you have "the question." Do I hold down this button, or do I just hit it quick?
Then there's the common "button delay" where you have a lag in the function of the switch.
Lastly, you have "button hitting syndrome" which can be caused by either "the question" or "button delay".....you just keep hitting that sucker harder and harder cause you can't figure out what the hell is going on.