I love to pull a ristretto and I also love a good normale. Is there any reason to believe that something half way between a ristretto and a normale is inherently worse than a ristretto or normale? Perhaps the answer is a clear no as we know (for example we saw in the brew ratio thread) the definitions of ristretto and normale themselves aren't fixed in stone, but perhaps there is some reason why coffee isn't totally stepless and that the whole range between a normale and a ristretto isn't just as good as the two extremes. Any thoughts?

It's an interesting idea, but as you said, it's very hard to even propose a mechanism for any sort of step. Here's one possibility, although I've never observed it.

Some coffees have a dead spot when roasting; for instance, Kenyas tend to pop at light roasts and dark roasts, and taste boring in the middle -- they have great acidity, and great dark roast flavors, but their middle flavors are nothing special. Something like this could translate into extraction as well - the right coffee may taste crisp under-extracted, and deep and complex over-extracted, but pretty much generic in the middle. In that case, an under-dosed lungo and an over-dosed ristretto could both taste better than a normale.

However, as I said, I've never seen a coffee do this (maybe because Kenyas still tend to be problematic or espresso at any roast).