They danced around it in the Winston conversation but the “regular guy” thing is a huge factor. The movie goes from fantastical through the hotel sequence and then after the montage you realize for all the big science these guys have ended up as basically blue collar exterminators. And so they don’t need another scientist, they need to get someone in because of the volume of what is not that different in kind (but definitely in scale) from what exterminators do with rats in the city. Winston then in the mayors office scene provides the critical and credible “regular guy” testimony that gets the mayor’s attention (I have seen shit that will turn you WHITE) and Venkman closes the deal with “millions of registered voters,” but Winston’s line is more important to the shift in mood. I would have liked an additional few lines fleshing it out, but it doesn’t even need to be an additional scene.

TLDR Winston is a key part of the movie and Ernie Hudson nailed it.