A strong and interesting A plot is what differenciates one fic from the next. Stories which are mainly B plot have a tendency to have a 'sameness' to them that, while enjoyable on one level doesn't always satisfy the need for diversity.
The best (we're talking longer stories here, of course) stories are ones where the A plot has a definite impact on the B plot and helps drive it in whichever direction it takes.
Using an example of my own... Serial Vengence could be said to have a fairly strong A plot, I think you'd have to agree that there was a fairly substancial B plot also. The events of the A plot essentially forced Lois into a situation where she had to confront her feelings for Clark much earlier than she would have and under much more intense conditions.
Vignettes and shorts, because they focus on single events, or incidents can get away with being totally B plot because that may be what the event or incident is about, which, in turn, actually makes it the A plot
Tank (who really enjoys the diversity that a good A plot can give to our, oh so familiar, Lois and Clark world)
