I wrote too much, and as a result, I think my point was obscured.
Basically, it’s this: Since the typewriter keyboard was invented, people have been told to press the space bar twice between sentences. Now computers come along, and they (and their human slaves) insist that people only press it once. Generally, computers should be designed to work for people, not the other way around.
It should not be necessary to refrain from pressing space twice to accomplish the (desirable) goal of proper kerning. If people like to insert two spaces, let them. If people want to insert one space, let them save the effort–although usability experts will tell you that the time spent retraining and having to think about this every time you finish a sentence will far outweigh any benefit. The computer should always be capable of doing the right thing either way.
Computers can be smarter than this! Sometimes (e.g., TEX), they are. Sometimes (e.g., Microsoft Word), they aren’t.