Death Knights were created as a weapon for Arthas. The corruption around them stymies from the powers used to keep them alive after death. The Sha are physical projections of emotion amplified to the point of becoming life threatening. By purifying the Sha you are not taking away corruption because it isn't actually corrupted. It is unbalanced.
The Sha as shown by their color scheme are pretty much the Yin/Yang balance thrown off balance. It could be argued that because Death Knights are supposed to harness hatred that they also are off balance, but personally I don't think that is the case. By reverting the Sha weapons to their original state, the wielder allows itself to harness both the Yin and the Yang side of the weapon in perfect harmony.
This expansion seems to be going off of the whole "Balance must be preserved" platform, with the Alliance and Horde constantly fighting, but always "staying equal", and heroes fighting the Sha to counteract their actions, so this is just my opinion on the situation derived from nothingness in my head.
Hmm, well that is certainly an interesting take. I might have to mull that over for a little while to fully grasp it, but you may very well bring up a good point... It does bring into question, though, as to where Death Knights stand in relation to the Sha. Considering that Death Knights, by their very definition and nature are consumed and indeed empowered by negative emotion, wouldn't they essentially be "Sha-batteries"?
Also, when I said that I'd like to wield an "Ebon Blade banner", I wasn't simply referring to a weapon made in the image of the Ebon Blade. I mean the literal banner they use, as you can see here:
The same blade that's on the Ebon Blade's Tabard, and seen near their many settlements in Northrend. Basically, I think Shadowmourne itself was little more than a cop-out, and there is a lot of evidence to suggest that it was given to players instead of Frostmourne *purely* because Warrior's could only wield Axes, because of Axe-specialization.
Frostmourne aside, I think most Death Knights would agree that nothing makes you really feel immersed in your Class as when you're wielding a true Runeblade. Considering we got nothing in Cataclysm, and very little even in Wrath of the Lich King proper, I think getting a new weapon-design would be well-recieved. Like I said, I quite like the thought of something with a bit of Lore behind it, which is why I suggested making it an "alternate Legendary" exclusively for DK's.