With the "commendation" system and vastly larger weapon & equipment inventories; why would you even bother to advance your Faction reputation?
Only for the armor bonuses to Strength, Discipline and Intellect?
Commendations ensure you must progress in Vanguard and Crucible to buy Legendary items if RNG isn't kind to us.
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If you want gear from the Vanguard, then you shouldn't be leveling a faction. However, if you don't really care about Vanguard and prefer the gear from the Crucible or a faction, leveling a faction is way faster than leveling the Vanguard or Crucible alone. BTW, if you level a faction you still get Crucible Commendations, so you can still buy gear from the Crucible and vendor.