With limited SP points early game disengage will not be used as often as strafe attack which could activate every turn. it also makes those annoying little ranged units a lot more manageable.
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Strafe 2 would come a long time after disengage 2, so taking strafe 1 instead of disengage 2 doesn't make sense. Strafe lets you attack more, but gets much less value out of each speed point. Combining lunge and disengage lets you kill far more stuff, with 3+ actions per turn, often keeping multiple speed points. I value dash attack more then strafe as it provides additional gap close and utility (ability to move through enemies). Even against Yendor I find I often prefer to chain disengage, lunge, dash attack, knockback (charge, power strike, cyclone strike), lunge, disengage, compared to just spending all the speed points on basic attacks with strafe.
I agree with you late game, but strafe attack is an invaluable asset to an early game duelist who can't afford to use their other (admittedly very powerful) abilities often. strafe attack allows one to stay mobile while continuing to doll out modest damage (+ you get that sweet strafe dodge) . While I agree that dash attack is much better, strafe attack has some good distance applications as well. I'll admit that strafe attack pales in comparison to many late game tactics available it is usually enough to trivialize most upper dungeon encounters (assuming that you have a bow otherwise it just makes melee more interesting).
If you have a bow I find upper dungeon is completely trivialized by that alone, no need for strafe attack. Additionally, a bow greatly multiplies the power of disengage 2: You can put everything into kill range, and keep 100% uptime on your SP by always getting the points back. A bow and disengage 2 is far more powerful than a bow and strafe attack in UD. Disengage 2 is all about letting you use those more powerful abilities more often, that's why it is essential imo for a duellist to get it immediately.