Mixing random team with arranged team is the most bullshitting thing in COH2.
Totally unfair.
They could have discord chatting, executing good co-op attack, while your random teammate can only do disarray.
I think they do it with the intention of getting people matches quicker. In general, I don't think there are many AT's searching at any one time so it would be just about impossible to get a match. Even when you do, you would likely play the same team over and over.