Actually, I have never had a bot harass me over steam. You just need to stop falling for those clearly fake friend invites. Protip: if someone named "hornygirl284" adds you, it's a bot.
Wait, was the rest of that directed at me, or him? Meh, either way, Steam is primarily gaming. And gamers are less tolerant to that kind of starfishry, so they don't do it. And in any case, AIM and other crap messengers get this, too. I'm just saying it isn't like MSN is the only place this happens, and I'd rather that not be a primary reason against it. It has tons of primary reasons as is.