One should probably note that most slabbing of ancients is of nice examples of fairly common coins. Athenian owls,
Aegina turtles, Corinthian colts. Lydian lions. (I really only collect Greek, so can't say about
Roman). Not the "esoteric" stuff of advanced collectors. But stuff that is pretty simple to judge for someone at all knowledgable. One problem is, is that the 'slabbers' do not guarantee authenticity. Secondly, being slabbed it is hard for the collector to examine the coin to judge for himself. Third, if the coin is broken out so the coin can be judged, then dealers or
auction houses may void guarantees of authenticity. But for some collector of American or World coins, who wants to have a 'iconic' example of ancient, slabbing might be a way to do that, and not worry about it that much. Storage, preservation, protection,
identification are all taken care of. Of course, for ancient collectors all that is
part of the fun. As Ute says, the certainty for the owner of a slabbed coin is a false sense of
security. But, for someone who wants to dabble, an example or two, they may intellectually know there is some doubt, but emotionally they are reassured by the slabbing. But, often one cannot eat just one chip, but one and one, and so on (potato chip metaphor). One slabbed, a little doubt but manageable, many slabbed and I would venture to guess the doubt can be cumulative.
I also think that the unslabbed collector, is often looking at their coins, and each time they sharpen their sense of what is authentic, I doubt that view slabbed coins would give the collector that kind of familiarity.