BSDF sampling versus light sampling
Edit: I suddenly understood the issue. The noise in the top right corner of (b) is because it's struggling to find the tiny fraction of angles in which the BSDF is not extremely close to zero. 


Yes your reasoning here is correct. 

Taken to the extreme if the top most surface was perfectly mirror (dirac delta BRDF), then it would never* sample the right ray from the light source that was perfectly aligned with the camera's view direction. For such a BRDF $\omega_i$ must equal $\omega_o$.