I use to think like this. I'm still sort of training myself to not think like this.
To me the logic was:
Weapon usage is divided up something like this:
Primary - 60% of the time
Secondary - 30% of the time
Exotic - 10% of the time
So naturally, I figured an exotic Primary made the most sense, and I stuck with that for about 3 weeks. Lately, though, I've gotten a couple of Legendary Primaries that actually feel awesome enough to use on their own. Namely, the elemental primaries from the Raid.
After I got those, I found myself using more exotic secondaries, but I've also fallen in love with my Final Boss that can shoot with walls. So at this point, I'm not even using an exotic, but as soon as I can get my hands on an exotic rocket launcher, you can bet your ass I'll be using it.