I think that's a good point to raise, alatar - and perfectly true. However, having just seen the battle again (beats 300 any day
), despite the invincible Dead forces, the remaining Orcs and Haradrim were still fighting and killing the Rohirrim and also inside the city, so it was important to kill all of the enemy forces as quickly as possible to minimize the damage, so the trio were fighting to increase the speed at which the Orcs were wiped out.
Also, they're warriors and it wouldn't look very honourable if they sat at the back whilst the slime did all the work.