In games, the point of burns or poisons(Dots) is to DO MORE DAMAGE over a LONG PERIOD OF TIME. The time is important because Dots are for long term fights or when up against a horde of low tier enemies.
In Destiny, Dots neither last long enough or do enough damage to have a meaningful use because PvP balance issues.
What should have happened with the Thorn in PvE is you take 1 shot per low tier enemy and watch them all burn to death. Same with Firebolts.
Destiny 2 must separate PvP and PvE balance
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One of the best features of Destiny is the fact that the guns felt exactly the same on both environments. It allowed the Guardian to practice new techniques on PvE, and then port them over easily to PvP. Edit: Who uses Thorn in PvE???