On Bitcointalk Forum there is an option to edit and delete self comments at any time.
Is this right if all users can change their message, even without showing when it was last edited?
Is opportunity of deleting self comments not the most helpful function for scammers?
Not at all , even delete there is record on admin panel. So if you find any supiciaus activity you can report to moderator if you have proof. Or if you don't have proof you can open an thread on Meta, so that moderator will investigate. But don't make thread if there is no seriuse case.
For editing that's why every post or every reply have " quote " option. You can use simply if you think there is possibility some thing wrong. If you queted than OP will not able to edit or delete your quote.
Here should be delete option. Sometimes some comment become useless. So OP can delete if he think it's not necessary now. Some one bumping thread , he should delete previous bump before next bump after 24 hour's.