when you parry you have to avoid parrying with the edge of your blade,
else the blade's edge will become useless fast
1) Not all offhand weapons have blades. Sai, tonfa and eskrima sticks for example, have no blade to be damaged.
2) Offhand weapons are routinely not slashing weapons. Some are, yes. But if your parrying dagger gets nicked, that's not going to be a problem when you stab someone with it.
3) Even if you are using a slashing weapon like Chinese butterfly knives, a bladed weapon repeatedly struck by a bladed weapon does not dull. It becomes
serrated. That does
not make it useless.
Also, shooting with any precision whatsoever with your non-dominant hand is basically impossible.
Depends. What sort of scenario are we talking about?
If you're talking going out to the firing range and offhand shooting a pistol into a target at 30 yards...yeah, that's going to be tough. If we imagine combat taking place inside an office or home, where distance to target is unlikely to be more than 15-20 feet at most, I think offhand firing is realistic. If we're talking a zombie apocalypse, or the previously mentioned "soloing the Mongolian horde" scenario, with potentially dozens of targets all at close range...yeah, I absolutely would want the second weapon.
shooting with any precision whatsoever with your non-dominant hand is basically impossible.
You forget about the magic that is called shotguns. Pick one up and you are a marksman. Doesn't matter if you are out of practice, shooting with the wrong hand or have the shakes.
I've never fired a shotgun with one hand, but I think if I saw somebody dual wielding a pair of sawed-off shotguns...I'd avoid that guy regardless of my estimates of his chances of hitting me.