In the perfect scenario, I guess.
In close ranger situations, I usually draw my pistol. So much more mobile then a full length rifle.
But, what it really comes down to is if you can take the extra weight unencumbered, and your play style.
I'm not a big guy, and I could probably stand to go to the gym more, so for me adding extra gear doesn't work.
It might for you though.
In my opinion, it comes down to what you want. Airsoft is supposed to be fun. I want to buy an M203 grenade launcher.
Are they that useful? Probably not.
Will it add a bunch of extra weigh? Heck yes, especially when you include a dozen shells.
But I'm probably going to buy one anyways, because I want one, and it makes me feel like a bad-ass!
It's better to light a candle, than to curse the darkness