It keeps gameplay fresh. I may not be one of them, but there are people who have gotten most of the exotics and desirable legendarys. Using the same gun, grinding the same strikes or story missions gets boring. Leaving the old behind and giving us new gear to hunt for, keeps the game playable. Yeah, I'm sad to lose some old favorites. But excited to find that new favorite. If I had the option to use the same gun throughout the life span of Destiny. I'd probably stop playing after I'd done everything the exact same way for months.