I have had a dual slot camera since the 1DmkII. I almost never used two cards even though I was a working pro. I did use two cards on occasion when I was very concerned about reliability but I never had a card failure.True...but, I strongly suspect that the great two-card slot debate gets a lot more attention on this forum than it does in real life. It would be interesting to see how many photographers actually set their cameras to shoot and save to both cards. I suspect it is like AFMA. A lot of photographers know they should probably do it, but very few working pros actually do.
I DID have card failures three times over the years but never total card failures with total loss of files. Rather it was single file corruption and as I had multiple frames I did not suffer a loss.
A few years ago I started using two cards all the time. I STILL have not had even a partial failure (shooting 2k-5k frames a month) but I feel very nervous if I venture out without a second card in the slot.
Anecdotally, I have heard about photographers claiming card failure as an excuse for not delivering images that were so bad they were ashamed to show them.
I have zero idea as to the frequency of this but I am confident it exceeds once.