Main reason is, that you won't ever stumble over those morons again when creating multiplayer matches or when you want to join one. If you've found e.g. a sandbagger, block him,
Not so. You're correct on all of your other points but blocking a player as outlined does not prevent them from showing up in a game search. If you're hosting you can obviously boot them but if you're not, your only alternative is to quit the match before it starts.
p.s., Yeah, I saw the dates but players are still believing wrong. :-)