But what if the attacker is using a longer-ranged weapon than a sword? A spear, for instance, or a rope dart? I wouldn't want to get close to a naginata with a single sword (two maybe), but if I could stay back and still attack I'd go for it.
Having trained with neither of those weapons, I am not speaking from authority here.
However, regarding spear . . . it depends if you're throwing it at the person wielding the naginata or using it as a short pole arm. Obviously, if used as a missile weapon, the spear has a definite advantage . . . provided you hit. Otherwise, you've just disarmed yourself.
If you're using the spear as a short pole arm, then the fight is going to be pretty even. In the purest form, a spear or a naginata is basically a quarterstaff with a pointy end. (Yes . . . I'm greatly oversimplifying . . . deal). So, there would be strikes with the sharp end, strikes with the butt of the weapon, blocks using one hand, blocks using both hands, etc. In that case, it comes down to who's more experienced/talented rather than one weapon having an advantage over the other.
With regards to Manuel's comment, the first time I saw naginata used in the SCA, the opponent chose short sword and shield. The guy with the nagi spent the entire match going backwards when not trying to attack, because the short sword forced his opponent to get inside his arc of attack. I think the short sword won, but that was because he wore his opponent down (that was in 1985 . . .my memory could be a bit rusty there.)