When it's their word against yours, medical evidence is what proves you're not the one lying. Medical evidence is what says that you are in pain, or you are blind, or you are mentally ill. It's what confirms that you aren't making things up or exaggerating. It's what tells the decision maker to believe your evidence over what the Atos assessor said, or simply to believe your evidence at all.
The government's line - repeated several times - is that these claims were fraudulent or cheats. Without directly saying so, Grayling implied in 2011 that because "the vast majority of new claimants for sickness benefits are in fact able to return to work", it is right to assume that many current Incapacity Benefit claimants are able to work - interpretation, many are cheats, frauds or otherwise manipulating the system.
With the Paralympics not all that far in the rear view mirror, Duncan Smith's outlook sounds almost consistent with the theme tune of that superb tournament: that people with disabilities are Harder Than You Think, and therefore don't need so many of your tax-funded handouts. It's a seductively simple premise, but it looks to be at variance with the facts.