A few years ago I received the projection for my private pension that caused me to opt out. My employer had massively downgraded the pension scheme for years before closing it and replacing it with an even worse one. When it got the the stage where paying the maximum I could afford for 38 years and 11 months would buy me a pension that would barely feed a dog (and no, that is not hyperbole - £21 a week would probably barely feed a dog in 2034), I opted out.
For many people, the state pension is the be all and end all. Company pension schemes are nothing like what they used to be. When I started paying in, I was due to retire with 38.9/60ths of my final salary and the AVCs I paid would have brought it up further. Now I would have to pay a much larger percentage of my wages for the same length of time to buy a pension that might keep a dog well enough fed. Maybe.