A grenade-launcher could be a good addition to the gameplay, but make sure it comes in handy in several occassions. I've played several games that just didn't require all weapons to be played so I sticked to some favorite weapons and they were sufficient for the whole game.
Some spots where the player has a height advantage would be very nice for a grenade-launcher, even better would be to have the enemies out of direct sight. Lobbing over obstacles and around walls would be great too.
So, good luck, you decide!