I was also taught to always use two spaces after the end of a sentence, to distinguish it from single spaces used within sentences. Even when using web interfaces that truncate multiple spaces, I still do this out of habit. I do think it makes a lot of logical sense in that white space is very cheap and wise use of it makes things much more readable.
On a somewhat related note, when coding I always use a ton of white space. Most coders seem to be into trying to make things very compact. I think it only makes it more difficult to read. Of course one can take things too far and make it less readable by using too much white space, but my general feeling is that as time progresses, people use less and less white space while the cost of white space is decreasing. We should be using more, not less.
Of course most everyone can read something even by leaving out lots of spaces and other characters. But I feel it's a matter of courtesy and professionalism that you care about the person reading what you write and your desire to have what you write read that drives you to format it in pleasing manner. This includes not only wise use of white space, but proper punctuation, capitalization, paragraphs, etc.
I know I'm much more likely to ignore something written like a cryptic SMS message than something using correct and properly formatted text without overusing/abusing abbreviations and shortcuts. But maybe I'm just old-fashioned.
