There was the LW 5.x then the 6.x and now the 7.x series in the last years, all of them got substantial improvements which made them incompatible...you can export to 5.x from both 6.x and 7.x. I really don't see the point of not supporting the 6.x format at least since it's the most popular format, I don't know much of my friends and studios that still uses version 5. Cheers!