I used to work for RAC AutoWindscreens and afaik they were free to people with Fully Comp insurance. I don't know if there is any costs added to insurance at renewal time, but Ive never heard anyone complain that it has put the price up.

Insurance companies prefer it as its cheaper than replacing the whole glass and probably only get charged £20 or so per 1 done. So quick cash for windscreen company and cheaper bill for insurance company.