Given that web apps should always redirect after a POST (or any non-repeatable request to change server-side state) ...
... how are people using MVC3 Model Validatio
What you mean by the "mandatory" redirect? Often we use a try/catch in controller, if try is successful, you can either redirect to a View(if you NEED to) or can also return any partial view, or anything you need. The catch is often redisplaying the original page with error message since the post request is not success.
Hope I did not misunderstand you :)