Set aside the SQL injection. Suppose there's a bug in Apache's path parsing such that using "\" instead of "/" causes it to interpret it as an escaped string, which somehow (bear with me) causes it to run exec("/bin/rm -r /"). Now some n00b comes along and uses "\" in the path, because he's used to paths on MSDOS; crashing the server. Whose problem is it? The client's, for sending a malformed request? How do you expect the client to know that the "\" will trigger a catastrophe? Or what if the client made a mistake, and while he thought it was "some query string" in his cut buffer, it turned out to be "; drop table *" (or something like that). Now whose problem is it?
If the server willy-nilly takes any input and doesn't check it, it is the server's fault.