Crucible has rare sparrows that can be upgraded, and imo, offers the cooler selection of sparrow colors. I personally bought the crucible sparrow that is yellow and green and upgraded it to legendary with the green upgrade from Xur. Factions are not PvP exclusive. They have specific bounties that offered on patrol, and you can earn rep through patrol, strikes, and weekly's. We're not asking for crucible gear, just faction gear. The factions do tie into the story with the grimoire as well. Why their gear can't be bought with vanguard is weird to me.