With pretty much every circuit in a home needing one of the two types of protection (AFCI or GFCI), why don't all new homes have combination breakers installed just to be safe? Would it hurt anything using both types of protections for every breaker? I know that based on code we technically don't NEED both types of protection, but is it necessarily a bad thing to have?
Of course this wouldn't apply to circuits that have exemptions such as dedicated circuits for an appliance.
There's no basic problem with what you propose. Feel free to put CAFCIs in all breaker positions.
You may have a problem finding AFCIs larger than 20/30 amps, and you may have trouble finding 2-pole AFCIs in some cases. In this instance, for equivalent protection where AFCI is not mandated, simply run those circuits in metal conduit, e.g. EMT/THHN wire. This will protect the wire, not the appliances.
AFCI breakers (which NEC now requires on new houses) make a lot of sense. Issues with wires going bad are pretty rare and there's not too many things outside that that can trip one. The likelihood of a nuisance AFCI trip is fairly low.
AFCIs also protect against hidden dangers. Arcing events can happen in hidden locations (like junction boxes) and you might not realize it until the fire department figures out what burned down your house.
GFCIs, however, are much more prone to nuisance trips. Imagine you dropped your hair dryer in a sink. Most GFCIs are next to the point where a grounding event can happen (i.e. inside an outlet). This makes them easy to diagnose and reset. If that GFCI is on the breaker, however, that may not be obvious at all.
GFCI might also be on something you don't want to be subject to nuisance trips. A kitchen refrigerator might get its own circuit, but what about a freezer or mini-fridge?
The final nail in the coffin here is cost. You're going to add a lot of cost to mitigate the small chance that somewhere not already covered by GFCI requirements is going to experience a grounding event.
