Here's the way I see it. First off, consistency is the key. Draw a line and never let your kids cross it, no matter the age. Personalities come into play. Some kids see a stern look in the parents' eye and they're crushed, knowing that they've disappointed. The next kid is a hell-raiser and needs a good swat on the ass to get his attention. It's also how the swat comes. If the parent does it out of frustration, anger, whatever, it's useless. The kid knows he got mom or dad to blow up. If it comes in a calm manner, he knows mom or dad has perfect control of themselves and are doin' what needs doin'. Consistency also means that mom and dad are on the same page. If the kid wants to go over to to his friend's house and mom says "no", he'll go to dad and ask. Dad needs to know that mom has already refused and must do the same. "What did Mom say?" "She said 'No'." "Well then, it's 'No'." Once the kid knows he can play Mom and Dad against each other, he's won the war. In the process, he'll get his parents pissed at each other which will lead to other problems.
For myself? My dad wasn't afraid to use his belt. However, I got a warning. "You ain't gonna do that again." If I refrained from "Doing it again", I was fine. If I did it again, out came the belt, every time. I knew what was coming if I disregarded my dad's warnings. That said, I'm now 50 and Dad is 84. I look after him and Mom as much as I need to as they're pretty self sufficient yet. I care deeply about that man and appreciate his hard work over the years to put food on the table.
So, in my case, the belt wasn't a bad thing. I don't think anybody's completely right or wrong in their views on this topic.