I think the point is it would become much less worthwhile to spam the site, at least with 'simple' spam. Stolen credit card numbers are easily available but using them just to spam Craigslist is probably a poor return on investment. Ironically, more complex scams could become more worthwhile if users start to show a higher level of trust in ads because they've been paid for.
You are probably right that increased trust because of payments could make the scams work better, but bear in mind that a typical stolen credit card can be bought for a few cents. They are almost free.
