Making a stat system would be silly because everyone would be pure 5/5's across the board. But then again, if everyone was a 5/5 across the board, everyone would be equally matched in speed/power/etc.
And anyone that's been in a real battle would tell you that luck does play a big part in it. You hope you're lucky that there's not a sniper that's been watching you in a firefight and takes a pop at you as soon as you peek over cover. You hope you're lucky that you don't get shot by a random ricochet round. You hope you're lucky enough that they didn't hide any IEDs on your patrol path. Sure, battles take skill, but luck is a very important factor.
And what does or doesn't need dice rolls would be agreed upon by the players involved. Because dice rolls would not be mandatory for all fights and only used when players agree to it, it's up to the players in combat/RP to decide when dice rolls are necessary. Or, if a GM is involved, then leave it up for the GM to decide whether or not a dice roll is necessary, just like in a tabletop.