If riff b builds upon riff a and the song continually evolves, it could be effective. I don't have any examples of this, just a thought.
I think what he's saying is that, like jazz, it's linear, not a structure.
riff A \ __ theme A
riff C /
riff B \___ theme B
riff D /
These develop into leitmotifs or their precursor, motifs. So you may see riff A several times, but often it mutates so that it's variations of riff A to complement the current theme. Riff A1, Riff A2, and so on.
This kind of structure is much brainier than anything you find in popular music, but metal and some electronic music come close. Fuck neofolk, it's garbage. So is jazz. Raise your standards or be slaves!