if you are only using mame, the 7th button is not necessary. it is mainly used so that you can have 4 buttons in a row to play neogeo games, and still keep the 6 button layout for fighting games. if you aren't a neogeo nazi, most people can play neogeo games using the standard 6 button layout.
if you are using other console emulators, then you may need more than 7 buttons