There have been several times I needed to edit a comment to include additional information or correct a typo. The only option (other then going behind the scene and altering the database record directly) is to add another comment with the correction.
Also some end users's support tickets are vague (don't explain the problem very well) their follow-up comments have similar issues. Currently (with version 4.0.268), we have the ability to edit the original support ticket the end user submitted, however we don't have the ability to edit their follow-up comments.
I know there may be some issues with allow technicians to edit end user comments which can be resolved by only allowing the administrator role to edit end user comments and not the technician role. This would resolve any conflict as each client would have the ability to determine who can do what regarding end user comments.