The "problem" concerning this and swashies isn't the uber fast weapons they already get, but the fact that certain damaging weapons might actually be useful in the off hand.
I'm really not positive of the complete impact of this on the weapons system, but it sounds like a large project that'll only band aid or amplify the problems that already exist in the stat layout. It would be interesting, at the very least, but I'm not completely sure it'd be worth the change. Why it wasn't implemented in the first place, well, I don't really know. I bet there's a decent reason for it, though.