And this is the bottom line.
Classifieds cater to sellers- end of story. People posting ads is what keeps things turning and if sellers don't feel comments are in their best interest, then it makes no sense to have them from Admin's point of view. No campaigning will change that.
As @Mountain Brusaid though, I personally would have comments for my ads and I really believe they benefit buyers. I recently bought a bike after taking a break from cycling for a few years, and despite 5 years of industry experience (many years ago), there were questions I should have asked the seller but only thought of after the deal was concluded.
Having access to other insightful "window shoppers" through their comments on the ad would have benefitted me in my decision making.
On the whole, comments favour buyers because they make it far tougher for seller's to deceive, exaggerate, lie or simply reply on a buyer's ignorance.
As is the case with used car dealers, complete honestly is rarely profitable. The best salesmen are masters at making the buyer feel like they got an incredible deal, which is difficult to do when there are comments pointing out that it is not, in fact, an incredible deal.