That's precisely the point. If you take 15 minutes to stop and farm, you've now added half of a strike just for a CHANCE at getting an engram vs. a guarantee. This is about a chance vs. the guarantee. Your guarantee takes 30 minutes. Your chance takes 15. In the time that you farm, I could be half way through another strike already, with the same % chance of getting an engram by killing things I actually HAVE to kill. If you want to stall for 15 minutes and smell the roses, do so at your own peril. Per the math, itself, the guarantee is superior to a random chance at farming an open area...