I disagree in that the FOID does or would make it harder to obtain a gun in IL if you were unable to legally get one. All points of sale and gun ranges all require you to have one.
Even in purchasing a gun through Armslist or similar, everyone requires you to show it in both buying and selling.
So a would be thief or felon would have to resort to other methods, illegal purchase or straw purchase.
Of cours, as my wife pointed out, they could just forge one and be on there way. I would assume that the forged number would be cought by a LGS, but not a face to face sale.