Most of your engrams will come from enemy drops before the Boss ever falls, not the end game roll. There will be many end game rolls where you don't get a single rare engram.
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that has changed you will always get at least one 1 rare engram and weapon/armor from roll, usually 2 or 3 engrams I've even seen 5
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What he's saying is that the easiest way to get legendaries is to complete A LOT of strikes quickly to get the marks to buy from the vanguard. That's how I got to 28 at the start of the second week. It's much easier then praying for engram drops.