Anything after mk2 was a bomb for midway until mkda.
Mkda removed the run button and used the "slower, more fluid" movements of mk2 in leu of the sf2-like combos used in mk3 and mkt (we don't count mk4 as it was sort of a test run for 3d and had it's own distinct style).
So even though this thread is WAAAY off topic I thought I needed to put my 2 cents in, being the mk king and all.
Money doesn't lie..... games with the run button in them didn't make as much money, once it was ditched they started being successful again.
As to if the run button is important for playing mk3... Well duh it is! Game designers don't just put an extra button on a cab to make it look pretty. That's like saying the "bomb" button isn't important on 19XX just because you can technically finish the game without it.
Stick a 7th button on there, as it's diagonal it doesn't take up any extra room and it comes in handy for a slew of games.