If the storyline is weak then most likely the games going to be weak as well, isn’t that what a game is? A interactive story. So If you’ve written a ‘Im going to kill everything whether it moves or not’ (GTA style) its fun for a while but after a bit you’ll find a urge to bond with a character, so see them grow and change. This doesn’t have to be in a positive way, the character could slowly become more bitter and twisted. But personally I want to feel like I have had an effect on the character. (Which I know is impossible but I like to delude myself) a truly awesome game doesn’t just make me feel like I have changed the character but that the game has changed and affected me.
If the is enough in the budget for a writer then one should defiantly write the script and better still find a writer with a interest or personal experience with the genre. A writer has been trained to write, a developed may have an interest in writing by 9 out of 10 times their not going to be nearly as good as a professional script/character profiler. Take Gordon Freeman from Half Life 2. He can’t speak, physically or emotionally react so why is he such a recognisable character? Well firstly the way the other characters react to Gordon and secondly because you become Gordon Freeman. This shows a good writer, being able to make you the player feel at one with the character and feel what the writer wants you to feel. The writer can make you feel good or bad, powerful or weak, just look at the best selling games there all about being the indestructible hero, Halo and the Halo suit. You can shot the enemies a couple of times and they die but you can take an entire clip and walk away. Doom is a prime example of this, your shipped to mars TO SAVE THE WORLD and not only that but your doing it ALONE. If that actually happened in reality you would either be brave and attack one of the monsters from hell with whatever you had closest (probably a frying pan or other similar household objects)and probably die. Or would you run away?