I normally don't do them often.
Rea the article: "I'll use isolation exercises for three weeks in a row and then not at all for a while (4-6 weeks)." ... in fact sometimes I can spend 2 months without doing any. And when I do it, it's not heavy.
I use them only when I feel that I'm less efficient at utilizing my abs in the big lifts (which does happen).
I would say that on average I use them 2 weeks out of 7. And I believe that there is zero risk to that. At one point we do have to stop being pussies. If we look at the inherent risk of all the exercises, we wouldn't do anything. The fact that I'm doing a ton o stabilization work for my core more than make up or any potential risk of crunches.
It is also my experience that to get the best 6 pack one need some crunches. I know that I have a visual improvement when I do them. Is it vein? Sure. But I think there are a lot more things I need to worry about that doing 3 set of crunches per day 2 weeks out of 7