Late to this but one reason for this being useful is if you'd been using an alternate frontend for MAME, had made your own list of favourites within that, and now want to migrate to LaunchBox. In this case, taking the txt file from e.g. MAMEClassic and working with that to replicate said list as a playlist in LaunchBox would be a far more efficient method of recreating it rather than going through your entire library again.
FWIW, I plan to just knock up a simple txt-in > txt-out prog to do this for me, was just trying to find if it's the ROM names needed for the playlist or the actual game names which brought me here.