I guess it could possibly be useful to indicate a bad
block on a
 tape, in order to preserve the numbering of all the blocks, and
 constantly giving a read error when used in a simulator. But it's
 kindof a weird reflection of a physical error into a virtual one. 
Unless you've corrected the error, and can therefore write an error free
image, _not_ marking the error somehow would be sort of cruel to the
next guy.  He might spend hours trying to _use_ the software that
contains a damaged file without any clue, else.
De