Mugzilla:  IIRC, Mortal Kombat and Street Fighter BOTH only support two players.
Thetered - Generally you are correct.  Only the Dungeons & Dragons games in MAME use 4 buttons, but a lot of games use 3.  No arcade games use more than 4. 
You might want more than 4 for console emulation, but you would be better not adding these to the cab, or using gamepads, IMHO.