On your point of never using underlines, I agree completely.
On your point of never using double spaces after a full-stop, I must disagree. Other users have given several good reasons for using double spaces, all of which I agree with.
In addition, I hate reading a professionally typeset, proportionally-spaced sentence which ends with something like "Washington, D.C." and whose next sentence starts with a proper noun. Without the double-space I can't always tell if the first sentence has ended. Just the other day I ran across such a sentence and had to stop and re-read it three times in order to parse it correctly!
Also, if all of these great typesetting rules have stuck around for a reason (ie, they work), they why is the rule about double-spacing suddenly bad? Didn't it work for all these years? Does it have no value?
Of course, you're quite possibly correct to draw a distinction between mono-spaced and proportionally-spaced type. I think that in my case, my eye is trained to expect to see double spaces in mono type, while at the same time is used to whatever standard is used in professional typography (single spacing, apparently). I see no reason why the two can't co-exist peacefully, esp. as a computer should be able to convert between the two. (BTW, can't typographers come up with a solution to my "...Washington, D.C. Bush..." problem?)
Even though I'm not a typographer, I still care deeply about this stuff. You can aruge about it until you're blue in the face, but I'll need a better reason to change my habits.
Incidentally, I was taught both rules (not underlining, and using double spaces) in school. Not that that proves their correctness, mind you, but it certainly makes my worldview a little more satisfying.