Keep in mind that the mix he is using contains a significant percentage of inexpensive berseem clover which is an annual along with the more expensive improved perennial clovers. While I think the reason they do this is because no one would buy there improved clover if they recognized the real price they were paying per pound compared to other improved clovers like Durana, WTI claim the reason is that the berseem pops up fast and acts as a nurse crop for the improved perennial clover. Oats doesn't have the allelopathic effect on weeds that Winter Rye does, so it simply takes up space the weed would otherwise occupy which is the same function the berseem is already performing.
The reason I suggested planting WR with it in the fall as a nurse crop is that the berseem may not reseed and come back in the spring when the nurse crops is needed. The winter rye will start growing again in early spring.
I understand folks do things differently in different places and what works well one place may not work as well in another. I'm just trying to flesh out the "why" a bit more so folks can weigh the options and select what they thing will work best in their area.
Thanks,
Jack