https://www.polygon.com/2017/9/8/16267766/destiny-2-bright-engrams-worth-it
So why isn't this pay-to-win? The Gunsmith sells random rare-quality weapon and armor mods for 2,500 Glimmer each. Mods seem to be Destiny 2's Glimmer dump, so think of these as being similar in value to ammo synthesis from the first game.
You can get rare mods from bright engrams, but they're so readily available from the Gunsmith that spending money to get mods is completely unnecessary, and any advantage it confers is so slight as to be immaterial. You get tons of Glimmer as you play — and these mods are not the primary goal of your grind — so you'll get all you need in the course of pursuing your other goals.