I think (my idea) it's a good idea to think about the future (and live today). I mean: Why buying lenses that overlap one-another?

Also: dslr's are sensitive to dust, so it's best not to change the lens with every picture you take., or at least avoid this as much as possible. Which is why a zoom can be attractive.

And: one stop halves or doubles the needed light. Yes: dslr's have the option to change sensitivity, just like having a set of films in your camera with a range from 100 to 1600 iso and being able to choose the one you want with each pic. But this goes at the expense of noise. Noise that will be most visible in higher sensitivities and low-light conditions. It's not necessary to dramatize this. But, depending on what you want to shoot it can be a factor. If you want to shoot indoors without flash, then it may be an argument to get a lens with a wider opening, say, a 2.8. If you prefer to flash inside, then this is not a problem.

Back to the start: if you already have a, say, 20-70, why buy an 18-85? Imo, that's wasted money. (once again: my opinion!). If you plan from the start, taking into account that whatever you buy, you have to multiply the "length" with 1,6, you can actually save money by buying what you want most, and, eventually add another lens later on to expand what you already have, not to overlap it.

There are no Borg. All resistance is fertile.