Another good summary of the arguments against AEI on this point. A couple of interesting tidbits.
1-Peter Wallison of AEI prior to the mortgage crisis criticized Fannie and Freddie because they served to retard the issuance of mortgages to sub prime borrowers. He's now the lead proponent of the theory that their excessive lending to sub prime borrowers is the problem.
2-Wallison's colleague at AEI, Ed Pinto, played 3 card monte in an effort to place the blame with Fannie and Freddie by saying that Fannie and Freddie had tons of sub prime AND OTHER HIGH RISK MORTGAGES and when you group all together it was a huge number. These so called other high risk mortgages had a default rate that was very close to the national average and much lower than the default rate of sub prime. By grouping them together though you are given the illusion that Fannie and Freddie had a lot of sub prime.
3-Wallison was appointed to the Financial Crisis Inquiry Commission and he presented these arguments. They were regarded as so outlandish by even the Republican members of the FCIC that these Republicans were asking each other if Wallison was on the take (which he is in a sense). We have their emails in which they wonder about this. The Republicans did have their own dissents from the FCIC findings but they would not join Wallison in his dissent and instead wrote their own dissent separately so as to be in no way associated with him.