The Fusion rifle is definitely a better fit for the Warlock. Thematically, the Sniper goes with the Hunter, the Shotgun goes with the Titan, and the Fusion Rifle goes with the Warlock. Thing is, the Fusion Rifle isn't a simple weapon. It's not something people would be instantly familiar with, and if someone is already relatively new to first-person shooters then throwing in a fusion rifle in the middle of their lesson would throw them for an unnecessary loop. That's why the shotgun takes its place.
English
-
But why not throw a new weapon at them to start? They would get used to it very quick.
-
Well, the theory is that they wouldn't get used to it very quickly because they wouldn't understand how it's supposed to be used. A shotgun is very obvious because it's a popular icon- same with the sniper. But the fusion rifle is unique. It has a charge time, which is unique. It's built for close-but-not-too-close range. It's just not a simple weapon, and you don't want to start people off with unnecessary complexity.
-
I find unnecessary complexity a good thing.
-
But if you were trying to teach someone something totally new to them, would you make it unnecessarily complex?
-
If they didn't try it now, they wouldn't learn how to use it. The first level is to teach new Guardians how to be, well, Guardians. Showing them unique weaponry now when they most likely won't die is better than giving it to them later when they can get shot down. Also, who doesn't love watching their enemies disintegrate/evaporate?