I'd say yes & no. If you're already done and gotten most of the exotics or a good bit, then no. If you're still grinding vanguard strikes to get your light level up then of course! A chance to get an exotic engram to just drop regularly from the boss is just great, fun, and interesting mechanic added to the game. Granted it's yet more RNG but it's practically like a mini nightfall each time but it's not guaranteed. And it only costs 7 coins for 5 tries at it. Then again you can save up twice the amount and guarantee yourself whatever else exotic Xûr is selling. Which is more fun though? Plus, Xûr sells exotics at 280 defence. Engrams give you 290 defence. Which is really worth it? That's your choice guardian.