I've probably only pushed it a handful of times in 30 years but that isn't the point, it is the ability to do it should one wish to.
Actually in modern CVRs the data isn't actually erased, the file system is scrambled so it is not possible to retrieve the data using the normal equipment airlines have access to, air accident investigators could still get the data if they really needed it in the same way the police can get files from erased disks etc.
If you have been making comments about the Chief Pilots dubious parentage or worse then its quite nice to know that your comments are going to be saved for posterity if a subsequent flight needs the CVR downloading.
I think it was the powerful US pilot unions that insisted upon the erase button when CVRs first came in.
If the same would apply to a video system I can't see why anyone would object to having it fitted.