I believe that a check was made in the past, and the main LDraw-based software didn't have a problem with spaces in filenames. Having said that, I have two items:
- The LDraw spec does not currently prohibit spaces, but merely strongly discourages them. As such, support for spaces in filenames in LDraw compatible software is required by the spec, and has been required by the spec since 2007.
- Users expect spaces to work, and MLCAD allows them without any kind of warning. Since users aren't going to read the spec, the discouragement there seems pointless.