ZuulMoG said:
One always uses two spaces after a period at the end of a sentence. That's just how it's done. If someone doesn't like that, they should have more of a reason than because they say so. I enjoy typing two spaces after a sentence. It makes me happy. It makes everybody happy. I don't see a problem. Is there any valid rationale behind this bizarre insistence on single-spacing sentences?
I can't tell whether you're being tongue-in-cheek or not. If you are, okay then. If you're not, I think it's fair to say that the "one" in your first sentence is not a correct use of the third-person impersonal

.
Personally, I tend to put two spaces after a period, because that's how I learned it, and when typing at 70 wpm, it's difficult for me to think about the punctuation and spacing. It just happens. I'm happy to rely on my computer to autocorrect it for me, and I'm very happy to adjust my autocorrect feature to match local customs.
The major thing I taught my students when I was a writing tutor was this: KNOW YOUR AUDIENCE. Presumably, you're writing in order to communicate concepts to the audience; and while punctuation can be noticeable when it's beautiful and part of the concept (see e.e. cummings's poetry for the classic example), punctuation and the like should never ever be visible as an impediment to that communication. Try to know what the expectations are for the audience to whom you want to communicate: if they'll notice your run-on sentences, or your use of archaisms, or your reliance on m-dashes, or your double-space post-period, then eliminate these things from your writing for that audience. By doing so, you'll communicate your idea more effectively.
This applies no matter the audience. I once tag-team-taught a ten-minute presentation on our humane society, alongside our director of animal control. He spoke very correctly and accurately about such things as the fact that "Our local ordinance, at least within our jurisdiction, prohibits allowing animals off-leash except when on the owner's property."
The poor six-year-olds didn't know what had hit them: they just stared at him with big round eyes, as if he were speaking Croatian. Eventually I stepped in and said, "That's right: if you have a dog, it has to stay in your yard or in your house. Unless! Unless it's on a leash!" Not quite as accurate, and not conforming to standard grammar (like this sentence), but I was speaking to my audience.
Know your audience, and tailor your communication to them. All else is salad forks.
Daniel