Yes- even single flavor mixes need to steep. They might not need to steep quite as long as more complicated recipes (though for reasons you might not think, I think) but they do need to steep.
I've spent some time thinking about why recipes need to steep at all (it's a bit surprising that they do, or at least it was to me,) and about how steeping changes them. I've come to some conclusions, some I'm fairly sure of, and others a bit speculative. One thing I am fairly sure of is that steeping is, in almost all cases we care about, mostly a matter of physical processes (rather than chemical reactions.) A steeped juice, compared to an un-steeped juice, is a juice with higher entropy.
In other words, a steeped juice is a well-mixed juice. The thing is, for many flavors (and it is why this matters for some so much than for others that I am most interested in and least sure about, though I have some guesses) this requires mixing on a very fine scale. Precisely how fine is another question. Given the means of mixing available to most of us, and the potential to do harm to our juice through more aggressive methods of mixing, we rely on Brownian motion to truly steep them.
Now it might be the case that if your flavors start off well-mixed they taste better even when they are not well-mixed with your medium. I'm skeptical about how strong an effect this has, but not _very_ skeptical. I can think of reasons why this might be the case, but...
It's at least plausible that a single flavor might need less steeping than several together, but if it's a flavor that needs to steep it needs to steep. I'm not just theory-crafting here- try Cap VCV1 (not my favorite, but many people love it) as a standalone at say 6% aged for 1 day, 1 week, 1 month, and 3 months, at once. I think you will find that each has its own character.