It's equally harmless as wintrading. As reedwise pointed out, in both cases there aren't any victims, since the ladder cannot be a victim.
Boosting does harm the ladder by reducing the amount of newbies getting their titles, yes. But its beyond dumb to compare it to real cheating / hacking. It's still a real player you're playing against, not a flying interrupthacking russian.
I used to love to queue into boosts - why? It gave good practice (and rating even), because it always meant you queued into good players and at the same time they kept the ladder more active. ( I quit before ladder merge, so cant comment on current activity)
I am not saying boosting is good for the game, but reading people compare it to DDoSing or any other kind of actual cheating makes my eyes burn.