Quality, uniformity, purity all vary more in combine run seed. Look at how seed that is certified and tested varies. Combine run stuff varies even more.

Purity is an issue, big time. If you have problem weeds in some of your fields, you can spread it across your farm even faster this way. 

I like clean seed that is as uniform as possible. That is the only way to maximize seed drop uniformity, improving the uniformity of stand emergence. Wheat is a crop that responds to a uniform emergence. Any crop that is off to an even start is easier to carry to a good, even, strong finish, and more so with wheat than most crops. Combine run stuff just doesn't emerge uniformly enough to suit that. Wheat that comes up the first three days after planting will outyield that coming up five to seven days by about 25%, according to some numbers i have seen.

I have seen growers swap a little seed around. I remember one getting Variety A from a neighbor, a very popular variety. Well, by April, it was obvious that it was some other variety. Well, the neighbor bought it from a neighbor who had said it was Variety A. There is no trail to follow, and who knows what it really was.

Combine run and non-certified stuff can certainly yield just fine, but the quality control needs to be there to maintain  purity and quality, otherwise you are going to have problems. 

Seed quality harvest is not the same as elevator quality harvest, and that needs to be kept in mind by the farmer.