Back to the question of why train against F-5's acting like MiG-21's?
There is a Human Factors Engineering concept known as schema. Basically a schema is a motor program in the brain that remembers how to do something. Now there are two types of schema, recall schema and recognition schema. Recall schema are what allows you to adapt to new things, such as going from throwing a baseball to football reasonable well even if you had never thrown a football before. Recognition shema is where you have done something before and you recognize how to do it, additionally you recognize quickly when you have done it right or wrong (basketball player knowing a shot if going in or not before it gets to the goal).
The F-5 acting at MiG-21 falls more into the recall schema, which gets better as you add variety. So by training against a lot of different tactics and aircraft the pilot adds to his recall schema which will allow them to adapt more quickly (speed the OODA Loop) and have a greater chance of surviving. Also if the pilot ever does face a MiG-21 he can use recognition schema.
Just thought it was interesting how something I read recently related directly to this discussion.