That's rumoured (neither confirmed nor denied) to be the reason why we're limited to two fire controls and for them ruling out a third - because of the need to keep it compatible with console controllers.
A rumour which has been explicitly denied already. Plus, since we never would get to meet directly within the game between XBox and PC/Mac CMDRs, and potential advantage from a 3rd or 4th fire button over a player with only 2 (due to physical limits of the controller) would be irrelevant, so even if that were the reason, it would be moot right there from the start.
Don't get me wrong, I would also like to have more fire buttons, but I don't believe there is some form of conspiracy where the controls were dumbed down for the consoles.