It's random luck. You're most likely to get one from a Vanguard legendary package or engram. Personally, I only use my exotic.
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I bought my first Legendary a few days ago just so I could have one. I want at least 1 legendary on each character since I have a Truth on each (had to buy those as well). The reason I want a legendary is so I can use Icebreaker or Suros or any other Exotic at the same time as a rocket launcher. On Crota I want to have the rocket launcher and Icebreaker at the same time. The reason I haven't considered luck is because in 3 1/2 months I think "Luck" or "RNG" would have at least given me ONE legendary Rocket Launcher...
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I was going to suggest buying the newest leg version. I have an Admonisher or Zombie Apocalypse to equip when using the Suros or IB as well.