imagine if early versions of photoshop could not open files created by the latest version !
You mean the way early versions of MS Office can open (I think not) .DOCX files created by Office 2007 ? ;) Ah, Richard beat me to it. :)
As far as the change in .MMO file format from MM 2004 to v5 is concerned, v5 included new features such as line styles which were not supported in MM 2004, nor the .CSV file format. So I think you lose any line styles you may have set if you use the .CSV format.
But as far as the incompatibility between v5.1.3 and v5.4.x is concerned, it might be something to do with the new .QC3 map file format, but I suspect it was intended to "encourage" users to abandon earlier software versions and accept the new License Management system. :evil: