Think about our situation this way. We are dead soldiers risen by a giant moon-sized entity called the Traveler. We have the ability to channel said entity's power for defensive and offensive purposes. Why? To fight the Darkness, the Traveler's opposite and enemy. The Darkness swept through our Solar System with the intention of total devastation and the destruction/possible consumption of the Traveler's power, the Light. The Traveler pushed the Darkness back at the cost of its own strength, and now we must protect it, fighting the Darkness's minions; Cabal, Fallen, Hive, and Vex. If we don't fight them, our only refuge will be wiped out, and humanity will cease to exist. Raids are our major blows against the Darkness. We've killed Atheon and conquered the Vault of Glass. We annihilated Crota, Son of Oryx, preventing the Hive from taking Earth. I guess you could count the no-longer-beating Heart of Darkness as a major piece removed from the chess game of life. We destroyed Skolas, Kell of Kells, preventing him from bringing the House of Wolves through time/unifying the Houses again. Soon I assume we will face Oryx himself. So what purpose does the Crucible serve in our endeavor? None. It is useless in our crusade against the Darkness. "But Viking, you Gjallarhorn-less scrub! We train via the Crucible!" You train by observing your real enemy, then practicing. Watching, memorizing, testing yourself against their strategies. Not by killing each other again and again. Hence, Crucible-dedicated Guardians are only in this for the fame and fortune, not to help our race survive.