Probably most people wouldn't want this. Or, you could just toggle it on and off. But wouldn't it be neat to be able to be able to select right from the game list each game's directory? Meaning, at least if only using one instance of mame*, no need for multi-mame or whatever.
*Of course, multiple mame selections would be nice, too. Say, one for single player configs, one for two player, different video settings, etc.