The Old Building & Loan wrote:
The problem with TOS's approach is that it often took what would have been a perfectly good half-hour episode and padded...it...out to fill that hour. TNG-era's A/B story approach seemed to be an attempt to improve upon this. I generally have no problem with it in TNG, which remains the most "comfy" Trek series to me...but by the time VGR and ENT had come along, they had problems holding my attention because it seemed like I'd seen everything they were doing a time or two before.
Actually, I think that the opposite is true. The B story, when used poorly, is the one used for padding. In fact, Q and the "trial of humanity" was written into "Encounter at Farpoint" for that very reason (to turn a 1-hour episode into a feature-length one). John de Lancie really saves the day there.
... Still, I guess that's better than when the A story feels like padding. "In Theory" comes to mind (A: Data and D'Sora attempt a relationship; B: the Enterprise
explores a cool dark matter nebula). Worse, the B story isn't even really connected to the A story. This random romance could have happened at almost time, such as when the Enterprise
was stuck in the booby trap in "Booby Trap" (another episode where the Enterprise
-D exploring something cool is the B story).
Of course, there are episodes with an A/B story structure that work well, such as BOBW which starts out like that (A: Riker realizes that he has trouble moving on in his career; B: something is scooping up Federation colonies). Of course, the A story perfectly sets Riker up as a foil for Shelby (and becomes relevant when Picard is assimilated), and the B story goes into overdrive when the Borg show up.
In general, the story structure really doesn't matter as long as the writing and acting are good. (Also, having a killer soundtrack doesn't hurt either.)