Because they're enemies? Seriously, ever one you kill, is one less that threatens the city. (Of course, the powerful ones can evidently resurrect themselves, no matter how your ghost tells you that you've set back the enemy by killing them.)

Are you retarded, not like they actually attack the tower you pleb

You evidently need to read your grimore.

And no one reads their grimore

I obviously did. Thus, your claim is disproved.

Do you want to pretend that if you don't kill every mob, then the game is over?

I don't believe I made that argument. You're evidently too much of a gamist to see a stimulationist perspective, much less grant that it has validity.

That's a no true scotsman argument. Yes, I see the fun in killing, no I don't see the point in taking your time when there's ten more years of this game to play

The True Scotsman fallacy is a specific sort of thing. If I were to say "No true Scotsman serves in the French Foreign Legion", it would be a definitionally true statement, and would not be a true scotsman fallacy, despite how much it resembles the fallacy in form. I was referring to the "threefold model" developed back in the mid-90s by Mary K. Kuhner. She posited that the appeal of games could be graphed on three vertices, "Story", "World", and "Challenge". She further posited that many gamers were drawn to a game by one of the vertices, and didn't really give a fig about the other two. These orientations came to be labeled "Narrativist", "Simulationist" and "Gamist". Rather than re-litigate problems Destiny has with Setting and Story, I'll use Alpha Protocol as an example, since it's almost a photo-negative of the issues raised. That game had a deep setting that reacted to your choices in believable ways, a great story, and gameplay/level design that varied between terrible and horrible. I freely acknowledge that the game was badly flawed, but I loved it anyway, simply because it nailed the parts of a game that appeal to me. It isn't so much that it's fun to kill enemies, as that my actions should have consequences. The Grimore strongly implies that there are a limited number of Cabal, so if I kill enough of them, I should run into them more rarely. Or if I'm told that my killing a Fallen leader has set back their cause by years, I expect the situation on the ground to change, and not to be killing the same frigging Fallen leader again the very next day (and get the same rah-rah lecture when I succeed).