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!

JKR did, grudgingly, show us a positive-sum Slytherin, which is to say a pre-Riddle Slytherin; his name was Horace Slughorn. Warning; essays on that site are addictive, and they will make you hate Deathly Hallows even more than you already ought to.

I'm not sure that it's so much what Slytherin contributes as how best to deal with the Slythery people in society. Best to put them in school to keep an eye on them, then to put them in their own House to keep them from bothering the others. Even if you would prefer to be rid of them entirely (as Rowling, at least, might be), that's not possible.

JK mentioned how Slytherin is not just the house of evil people, but that each evil person came out of Slytherin. I do not remember what other positions Slytherins have in Canon but we can surely come up with reasons to have such a house. First it helps students to reach their full potential (at least in the theory that fiction is), second it provides a training ground to have people for the more dirty needs of society like leading in a war. Was there ever a mention which house Dumbledore went too? Third it provides society with some training for how to deal with evil people. If there are none left society gets overrun by an outside force. Fourth there is the value of not having the Slytherins poisoning other houses. Having a house of Bullies would be a nice add to the real world (till your research shows that a strong hierarchy forms in every social group.) Fifth it gives you someone to contrast yourself against. Sixth if Slytherins are more adventurous, prone to doing dangerous things that they are also the inventors of new things. A healthy society needs some innovators, some bureaucrats, some workers, some people to provide Emotional support, some teachers and so on. I remember reading some reasons for the House of Slytherin in Canon, but memory evades me.