Sieve is a mess. The rewards are not worth the effort. Once you've gotten the exotics, there's no point. It's designed to waste as much time as it can, the matterspark phases, the vex shields, the bullet sponge ; teleporting ; invisible pain in the -blam!- bosses. then if you get the special vex phase, everything becomes a bullet sponge. Then there's the atrocious timer. They wanted it to be "rare" like vex incursions (which was actually a dope public event) but wanted in predictable, you an see they've tried to remove a lot of randomness from the game, making things more predictable,like ammo generation. But considering this falls very short of the fun of vex incursions and other large-scale public battles, the rewards are weak, save the exotics, and it's one of the marquee activities for this season. Putting such a restrictive timer was silly and done just to throttle access to the exotics.