In all the years I've been collecting Ancients, I have only purchased one in a slab. When it arrived, I broke it out immediately. That was a waste for someone to pay to have it slabbed in the first place. I do not even remember which coin it was, but I'm fairly certain it wasn't a high priced coin. I might consider slabbing a 20k coin, but for my collection and my hobby interest, slabs will never be worthwhile, let alone tolerated.
To be fair, I keep one ancient slab. It lists the wrong emperor, wrong mint, wrong dates, and uses a grade unknown to ancient collectors, (AU). I keep it around for comedic relief, and bought it from Frank Robinson, (who pointed out the flaws on the slab), for the same reason. But, just like Chinese counterfeiters, slabbers products will get better, but will never get "good" enough to convince me to wish to own them. To me, the main use of slabs in ancients is for the numismatic equivalent of telemarketers, selling overpriced goods to undereducated buyers.