When do you ever need more than 4, maybe 5 shots at max? When you face multiple opponents at once, especially in 1v2/3/4 situations. By limiting the Gnasher to six shots (instead of 8) you make it even harder for lone wolf players to carry their team.
Why would you do that? It’s one of the biggest flaws and one of the issues Gears of War has that it is off-putting to solo players. You don’t have to actively make things worse, TC.