They aren't guaranteed. I ran three Roc strikes on Saturday and walked away with two blue engrams as a reward for one, and nothing for the other two. I wasn't impressed but knowing what the game's RNG is like tried to remain philosophical about it.
Grinding through the game is the most effective method of getting rewards. You can buff these chances by playing the harder strikes (Nightfall, Weekly etc), but it is still grinding that increases you reward chances.
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Finishing strikes is more of a guarantee than killing hundreds of monsters in the hopes of getting a blue or legendary engram
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Very true, they do provide a much greater chance of loot drops. However, by skipping past everything, you are guaranteeing you won't get those other potential drops. You are focusing on the belief that those rewards at the end [i]must[/i] be better than the possible rewards gained whilst getting there. However, I've had plenty of useful rewards drop during the route to the end. Many Strange Coins, many useful weapons and armour. Racing past everything may reduce the strike to about 10 minutes (at best). By not racing past, it's likely to take 15 minutes (or so). It also reduces your chances of reward.