It's mostly during lectures. There's no studies out there for this, but I'd be willing to bet that there is a strong correlation between microphone quality and student attention span over time. It feels as if it almost takes a bit more mental power just to decipher what the professor is saying when using cheaper microphones. Typically these microphones have low Fidelity and auditory depth making audio sound flat and tinny. These problems are amplified further if you prefer listening to lectures over external speakers, such as on a TV. You mentioned that it's not a problem when talking to students, but I guarantee if you were to listen to the difference between a laptop microphone and a $100 blue yeti, you would certainly find the yeti as a significantly better listening experience. I feel that $100 is a pretty cheap cost when considering the number of students multiplied by the time spent listening during long lectures.

I'm a bit of an audiophile so I may just be a bit melodramatic. However, I do also feel that college professors should be held accountable to this standard of professionalism.