From what I've read, the original bug involved malware already installed on the PC using the Steam client to run other code. While I'm not a security expert in any way, that doesn't seem to me like a huge exploit. If the attack requires installing malware on the victim's computer, why not just do the evil stuff directly with that malware? If that's the case and I'm not just remembering it wrong, then I could see why…
It was a priviledge escalation. An attacker could go from running as a compromised user to running with system priviledges.