My ViewModel has validation attributes that ensure that it wont be empty etc. - Is that enough or should I let my the code contracts I have made be in the ActionResult too?

Example:

// CreateCaseViewModel.cs

public class CreateCaseViewModel
{
    [Required]
    public string Topic { get; set; }

    [Required]
    public string Message { get; set; }
}

// CaseController.cs

[AuthWhere(AuthorizeRole.Developer)]
[HttpPost]
public ActionResult Create(CreateCaseViewModel model)
{
    if(!ModelState.IsValid)
    {
        // TODO: some cool stuff?
    }

    if (string.IsNullOrWhiteSpace(model.Message))
    {
        throw new ArgumentException("Message cannot be null or empty", model.Message);
    }

    if (string.IsNullOrWhiteSpace(model.Topic))
    {
        throw new ArgumentException("Topic cannot be null or empty", model.Topic);
    }

    var success = false;
    string message;

    var userId = new Guid(_membershipService.GetUserByUserName(User.Identity.Name).ProviderUserKey.ToString());

    if(userId == Guid.Empty)
    {
        throw new ArgumentException("UserId cannot be empty");
    }

    Case createCase = _caseService.CreateCase(model.Topic, model.Message);

    if(createCase == null)
    {
        throw new ArgumentException("Case cannot be null");
    }

    if(_caseService.AddCase(createCase, userId))
    {
        message = ControllerResources.CaseCreateFail;
    }
    else
    {
        success = true;
        message = ControllerResources.CaseCreateSuccess;
    }

    return Json(new
    {
        Success = success,
        Message = message,
        Partial = RenderPartialViewToString(ListView, GetCases)
    });
}