How worthwhile is it to install dual-function CAFCI / GFCI outlets rather than going with ones that are just GFCI? Is it a significant increase in safety factor?
They protect from different things.
AFCIs are primarily about preventing fires started by electrical arcs. They were (IMHO) not ready for prime time when they first hit the market, I think they have gotten better (certainly fewer "my vacuum won't run on this circuit" stories these days. But not zero, I see on a quick look.) If you happen to be in a jurisdiction like Chicago that runs everything in metallic conduit for the same reason, not much incentive to change to them unless required. Contrariwise, if you have doubts about the integrity of your wiring, they might be a Very Good Idea, even if not required.
GFCIs are primarily about (not) shocking people.
AFCI and GFCI are both zones of protection. They are not receptacles, though they are sold as a "combo device" including that function and a receptacle. Those protect their own sockets obviously, and they can protect a downline.
With AFCI, pretty much the entire point is protecting the downline wiring in the walls. There's very little point providing AFCI at the receptacle, unless it's in a bedroom where it can protect an electric blanket or plugs that tend to get smashed by furniture.
In fact, AFCIs in new construction aren't even legal unless they're a) at the breaker, b) at the FIRST receptacle AND the wires are carried inside metal conduit or embedded 2" inside concrete.