doctorbadwolf
Heretic of The Seventh Circle
I agree that it is the simplest solution, but I’m of the opinion that the simplest solution is only the best solution if it isn’t worse than more complex solutions. Allowing dual-wielding with only one light weapon and one non-versatile weapon opens up the possibility of rapier and dagger, yes, but it equally opens up rapier and shortsword, which is strictly better than either rapier and dagger or dual shortsword. Heck, it’s strictly better than greatsword until you get extra attack. Adding the ability to be dual-wielded with a non-light weapon to the dagger is a better solution, but it also makes the dagger the objective best simple weapon, even more so than it already is. That may or may not be an acceptable consequence to you, YMMV. Personally, I would rather attach the ability to the rapier or make up a new weapon to attach it to.
In other words, Occam’s razor tells us that, all other things being equal, the simplest solution is the preferable solution. All other things are not equal in this case.
Easy concerns to address without adding weapon properties. The rules for TWF should be contained in the combat section, and in the abilities that center on fighting with two weapons, not in the descriptions of special weapon traits. Referencing light weapons or versatiles weapons or two handed weapons is one thing, because you don't need to read the trait description, you just need to look at the chart, and see if the weapon has that trait.
I'm sure we could word the rule to allow daggers without adding noticeable complication or breaking simple weapons.
Honestly, TWF needs a boost, so allowing any light weapon in the offhand, regardless of main hand, or just specifying in the rule that rapiers are the exception, rather than daggers, is fine, but the game won't break if it also gives a +1 AC or Attack when your second attack is with a dagger. Shortsword is higher damage, dagger is either better defense or more accurate.