Of course almost all mutations won't be beneficial (this is also true of germline mutations). But "can only be harmful" seems to imply that beneficial mutations are impossible.
agreed. sort of my point: some changes can be beneficial, but the process is very risky and the cell goes through great measures to minimize any sequence changes. some viruses are sloppy on purpose, as they have tens or hundreds of thousands of offspring, so trial and error is acceptable.
