Matz says, on January 28, 2009 (EST): 

The bang (!) does not mean "destructive" nor lack of it mean non
destructive either.  The bang sign means "the bang version is more
dangerous than its non bang counterpart; handle with care".  Since
Ruby has a lot of "destructive" methods, if bang signs follow your
opinion, every Ruby program would be full of bangs, thus ugly.


And I (David A. Black) say: 

So please stop writing bang methods that have no non-bang equivalent!
The bang in isolation literally means nothing. It's purely ornamental,
and dilutes the conventional usage (see above) to the point where it's
unrecognizable and pointless. 

(Exception: things like Builder, where the bang/non-bang distinction
is completely redefined, and documented as having been redefined, for
the sake of domain-specific semantics.)