In theory, yes, balance druids would want to switch between both sides of the spectrum, it's just that the base system doesn't support that fact unless something artificially influences it. Going from one end of the spectrum and then swinging to the opposite end of the spectrum would end up with the getting fewer buffed casts off and more casts off with a lower buff (assuming that the variable of the buff depends on how far over to one end of the scale you are.)

For this reason, you would hover at one end or the other in order to maximize the number of casts you get with a bonus and the potency of the bonus. Conversely, there are other alternatives. If the buff if flat - meaning that you get the same buff value no matter how far along on one side that you are - then the balance druid would merely hang around the middle of the spectrum, but still do the same thing.

Without some specific interaction from Nature's Torrent, the basic system of Eclipse, as far as we know it anyway, just doesn't support a cast style where you actually switch from one side or the other. However the system works out and how Nature's Torrent influences Eclipse is obviously going to be the major factor in how our rotation ends up turning out, and it is true that we lack any real solid information on any of this.

The only thing we do know is that the current RNG of Eclipse is gone. We will, potentially, still have some Eclipse RNG to a degree in so much as how far a spell is capable of pushing the bar, but it won't be anything compared to how it is today.

Nature's Torrent probably won't be our answer to movement DPS, it looks more like that will be Wild Mushroom. Wild Mushroom is instant, so it can already be used on the move, it also doesn't have a cooldown, so it can be spammed endlessly. Blizzard has said that a druid will be able to force the mushroom to explode when they want it to which is most likely to be done by casting the spell again (and there being a limit of only one mushroom active at a time.) So, you could merely spam Wild Mushroom on the go. Obviously it won't be great DPS, but it will be better than nothing. WM is an AoE attack that Blizzard has said will find a way into our single target rotation. Without a cooldown or the like, the only real feasible way that this is possible without the ability merely dealing more damage than Wrath or Starfire is for it to be our movement spell (unless there is some other new talent that they add which can fix this (such as the explosion adds on a hefty DoT or debuff of some form.))