This thread is really a tech log discussion, not testing. Interesting nevertheless, though.
I think John T didn't emphasize Cd curve enough.
While I didn't fly the 727, my recollection of the events of the time is that the drag could increase very rapidly as speed dropped on the 727. This was partly due to the swept wings, but also due to the very effective (for the time) high lift devices. The drivers had typically come from DC7s, and were not used to such behaviour.
Indeed, I recall hearing that the 727 could fly slow enough that full power (which was spectacular compared to the prop jobs) would not maintain level flight and speed.
Slow engine spool up was also a factor.
Swept wings with low aspect ratios tend to develop high spanwise flow at low speeds, and consequent interesting tip vortices.
Another example would be the F102 delta of the same period. My recollection of some aerodynamic notes circulated by command was a data point that although the F102 could achieve an instantaneous 5g at only 300K, it would then decelerate at 60K per second, producing dramatic results.
With reference to the F104 discussions, that was an aircraft with very high wing loading and short wings, and the gliding characteristic of a brick. Someone pointed out that some very skilled pilots have successfully dead sticked 104s. However, at the OTU the instructors convinced most line pilots not to try it by demonstrating a simulated flameout circling approach. The drill was to ensure arrival overhead at 18000 feet, circle in a rate 1 turn with gear and flap up, and put down the gear at least 3 and not more than 6 seconds back. (Also, on a normal approach middling engine power was required to operate the boundary layer blowing: cut the engine crossing the threshold would result in eye openiing gyrations in the flare.)
By comparison, the T33 could do an flameout circle from overhead at 5500 feet with gear down all the way.
Really, however, the primary reason for powered flares would be the high drag and consequent rapid speed decay at idle. Swept wing a/c are not alone in having these characteristics.
As a final teaser, it would be interesting to speculate whether the new types with large winglets have Cd curves which don't go up as rapidly with speed loss as do their cousins without the winglets.