I agree with most of the comments above, and I also think that people new to the hobby, particularly those coming from collecting moderns, are often a bit overwhelmed by it. Many have assumed that all ancient coins were rare and valuable and could be seen only in museums, and are therefore very suspicious of them. They are afraid of getting ripped off, either by buying a fake or by paying much more that what a coin is really worth. So slabbed coins offer some degree of assurance that at least the coin is real and not fake. Of course, the irony is that while buying a slabbed coin reduces your chances of buying a fake--although it does not eliminate--it increases your chances of paying too much for it.
I'm not questioning the veracity of yours or anyone else's statements. I'm just trying to understand the reasoning. Chris
Heritage and Great Collections slab virtually all of their coins, presumably to entice collectors of moderns who are conditioned that coins must be in slabs to have maximum value. I suspect other dealers who primarily trade in modern coins also send in ancients for slabbing because it is easier to sell them to their customer base that way, and having a TPG slab the coins means they don't have to educate themselves about the "product" they're selling (and that's what coins become when slabbed... products, commodities, merchandise).
Most of the ancients in upcoming Maison Palombo Auction look like they where slabbed right after being struck! For me, its really hard to fathom how these coins are in FDC condition. My feeling is that there where collectors even back in 350BC that preserved them for us today. Even that unique Saturninus Aureus in NFA auction was a perfect coin. However, slabbed ancients do have details like smoothing/ cleaning/ clipping/ envirnomental damage. MS GEM 5/5 5/5=CNG FDC grade. John