1) Jeez, what the heck am I going to do with my diseased units if I can't use them as archer decoys? I thought that's why diseased units take a year to die, so you have time to march the dying and wounded to the front lines.
2) The AI is pretty good at not wasting gems too much. I did like it one time when I assassinated a Prince Of Death with a Disease Demon, and the Prince of Death obeyed his script to cast Darkness.
And sometimes even scouts have a reason to attack/retreat: if you're at war with a glamour nation, that's the only way to see what's hiding in the provinces.
5) This one's on the bug shortlist now. I think if it's shortlisted as a bug then it's abusive to use the tactic (eg. sneaking non-stealth units out of castles, etc.)