I don't see why they'd have to do that. There's no requirement for me to play through Assassin's Creed 1-3 to play Assassin's Creed 4.
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I have to ask if you've leveled a character recently. You learn next to nothing about your class from leveling and LFD, and nearly everyone skips battlegrounds if they're not kitted out in heirlooms. Most of the nuance of any classes I've played, I picked up at max level.