The Roto-Swirl, although there was a large capacity version for a few years until the mid 70s, was primarily a standard tub agitator. The large capacity machines, that Sears first called 'Jumbo' capacity, are the models in which the Dual-Action earned it's keep. There are some specific challenges in moving clothes from the top of a large tub to the bottom which a standard agitator doesn't have to contend with. These large capacity models were quoted to be 38% larger. That is translated from being several inches wider in diameter, and three inches taller. That may not seem like a lot of difference, but it is massive.
In the 1980 and older models, a standard Roto-Swirl is too shallow and can get overwhelmed (water would rise to the very top). Most of us here know better than to overload a machine, but the DA was designed to address the average user's propensity to do that. The large capacity version of the Roto-Swirl was a basic super Roto on Steroids, but the machines used transmissions with the same internal parts. Sears and Whirlpool found out that the drag caused by the massive agitators and a heavy load of laundry could prematurely damage or wear the transmission, so agitators that could fit the big tub, not damage the trans, yet still be effective had to be designed. That is how the Penta-Vane and Penta-Swirl came to be. These agitators have five shallow vanes (less drag) vs. the original three taller ones of the big Roto-Swirl. These agitators also allow a common agitator shaft to be used between all models.
Those two agitators were considered OK unless a user stuffed the machine, so the DA was developed to help move clothes from the stratosphere of the tub down to the vanes. We never complained about the Penta-Vane in our 1974 Kenmore, but my mom and I are not overloaders. I didn't like the DA in our 1983 Kenmore at first because of its lack of convention, but I came to admit and agree that they do a noticeably better job in keeping full loads mobile in those big tubs. The DA has four flexible vanes, which have some "give" to them which helps with the big-tub transmission stress.
A DA doesn't do much good unless the machine is on high water level and full of laundry, but that's where it shines. There was a standard tub DA, but they were not common and probably nowhere near as necessary, since you're right - the Roto-Swirl was sufficient in a standard tub machine. I recently acquired two standard tub DA machines - one has a 1976 ratchet style design, the other a later one. Eventually I'll find out if these models offer any benefits over a Roto-Swirl or Straight Vane.
