I have an error where a new library didn't know what some notices were for so they deleted them. Now if they get a hold and try to print the hold slip they get: Software error: Could not find a letter called 'HOLD_PRINT' in the 'reserves' module at /home/koha/kohaclone/C4/Reserves.pm line 1914. I don't think that any notice that is installed with Koha by default should be allowed to be deleted since they're all there for a reason.
I can imagine some cases where they could be deleted. For instance if you create specific letters for each branch, you don't need the default "all libraries" letters. Or if you want to be sure the letter won't never use.
I agree with Jonathan - For example there is no other way to turn off the suggestion related notice templates than deleting them. And some notices are not strict about the code - like the overdue notices can really use any code, as long as they are defined in the notice triggers. We'd probably need to make only some non-deleteable. Or have a nicer error message telling the librarian to create a new notice with code X instead of having Koha explode.
Quite a few important slips cannot be deleted now in master
*** This bug has been marked as a duplicate of bug 17355 ***