I personally find that frequent ab work is unnecessary if you do a lot of compound movements.
I never used to train them at all after hearing that bodybuilders never train them directly Roll Eyes I then began to wonder why I had no abs despite being reasonably lean.
I never used to train my abs either, feeling the same way as the 1st quote above. I heard endless guys talking about "if you do compound movements, or regular movements heavy enough, the abs get enough stimulation and you do not have to do any direct ab work." That theory never worked for me.
It wasn't until I started last years lean out phase, that I realized the above never worked for me. So I started training abs and they're better than before, but still not where I'd like them to be.
Oddly enough, I use to carry my amplifiers around and have to do twists and turns with them, and my obliques/abs were pretty damn good then. Maybe isometrics are good for me?