A lot of programs (even commonly used ones) fail miserably if you try to give them file names with spaces in them. The most common way to fail is to pass filenames to the shell unquoted. When you try to make people fix these issues you usually get the same response:
Not fixing, just rename all your files and dirs to not have spaces in them.
This is both understandable and totally misguided. The point of failing on spaces in filenames is not about those files. It's about not properly sanitizing your input and output. To see why this would be a problem, just imagine what would happen if you passed the following as a filename to a program that will use it in a shell command invocation:
; rm -rf /;
Because of this every case of an application failing with spaces in file names should be classified to the same severity level as an SQL injection vulnerability.