Surely you're overlooking Slytherin's positive qualities as defined by MoR. Slytherin are focused on manipulating people, concerned with power, and quite cunning. If you want to keep fooling the muggles, have good PR guys, and keep alive the dangerous, secret lore that other houses would consider too evil (so we can use it to fight aliens), you need Slytherin.

It's the only house that has consistently churned out people who actively work at defeating death!