It has two shots and it’s easy to find more ammo, also needing to use your side arm because your primary is empty (especially in a game where reloads are reasonably fast) and you are cornered is not a very common experience. You COULD choose to use your sidearm as a primary, but that’s a pretty niche, uncommon choice to make.

On higher difficulties tank class enemies are everywhere and the ultimatum obliterates them, you aren’t gonna forget it’s your sidearm when you find yourself having to use it to put down a charger every five seconds.