The nitojutsu of the Hyoho Niten Ichi Ryu is built around using a katana in one's main hand and a wakizashi in one's off hand. It is hardly the same as wielding two longswords at the same time. Furthermore, much like the so-called "Florentine" style of fencing the entire premise of the school is about using the smaller off-hand weapon for defense and parrying. It is not generally used for getting in "extra attacks".
Actually, if you will read his Wind chapter, you will hear his opinions on why longsword and shortsword is preferable to two longswords, which I agree with. Furthermore, you never wield a weapon in your second hand if it is purely for defense; you would be better with a shield or some kind of fork. Musashi's style never, ever assigns any movement or technique as being purely defensive.
In general, in D&D, to hit is king. So it actually makes a lot of sense for a fighter in D&D with a rapier and dagger to use his two weapon defense and only make one attack a lot of the time, particularly if he has rogue levels and uses Improved Feint. But attacking with both is still an option. To me, that's pretty much how it should be.
I am less concerned with having two attacks as the concept of having two weapons should cause you to win more often against someone with one weapon, one-handed weapon. It is true, fighters are often worse off with two weapons than one. But in general, I think that's because they have a specialization in one-handed style; they don't fight badly with two weapons, they simply don't have the advantages of a specialist. I always encourage newbies to learn one weapon first, then two. but it's not because they can't wield two, it's because it becomes a crutch. Newbies with two weapons are simply less safe, as well, and depending on that second weapon to be there can lead to some bad fencing habits. But in general, someone with a few weeks experience is going to be better with two weapons than with one. Some people just can't do two weapons without special training, but that's not most people; some people can't do two-handed weapons, either, and we don't slap a on -6 penalty to use one of those.
Using two weapons as an advantage over one weapon should be an option for any character with proficiency with those weapons, not simply those with special powers. Using a sword and axe is nothing at all like being able to shoot an energy bolt from your fingertips or vault over someone's head in combat. What's next, making extra languages a Power of the bard class?