Counterpoint: Working your core is never a waste of time 💪
Sexist counterpoint: strong obliques make it easier to hold babies and children.
Discussion
Why is that sexist? Women have babies.
But to explain my point.... Working on the rectus abdominal tends to result in stronger obliques also. Focusing only on obliques results in a wider waste. My opinion is to do the exercise that benefits both because they work together.
I don't actually think it's sexist, but for whatever reason making an argument centered around the fact that women have children is seen as such.
But you make a good point. Compound exercises are by far more effective, and if you can work your obliques alongside others, everything in between gets stronger too.
Amongst other things