Actually the man is debatably the least competent member of the team - Kylie knows about the paranormal in general so she's a stand in for Ray, Ortiz knows about handling weapons so she's filling Winston's shoes, and Janine provides the team with Venkman-tier snark, while Ron is technically just a poor man's Egon, a blowhard who's proficient at neither the actual Ghostbusting or the maintenance of the equipment. His biggest achievement in the series was to make a cheap knockoff of the Ghostbuster's equipment, which brings the team more harm than good.
It works because these are all characters who were introduced previously, they've had contact with the Ghostbusters and have seen them do their job. They're not bumbling idiots picked at random, they have a basic idea of what needs to be done in order to properly bust a ghost, and while they lack the experience, they most certainly can fill in for Venkman, Stantz, Spengler and Zeddemore while they're gone. The reason it's so great is because it brings new blood to the series, characters with entertaining personalities and quirks, all while being respectful of the continuity and the old characters the fans know and love.