I was watching Rob Connerys webcasts on the MVCStoreFront App, and I noticed he was unit testing even the most mundane things, things like:

public Decimal DiscountPrice
{
   get
   {
       return this.Price - this.Discount;
   }
}

Would have a test like:

[TestMethod]
public void Test_DiscountPrice
{
    Product p = new Product();
    p.Price = 100;
    p.Discount = 20;
    Assert.IsEqual(p.DiscountPrice,80);
}

While, I am all for unit testing, I sometimes wonder if this form of test first development is really beneficial, for example, in a real process, you have 3-4 layers above your code (Business Request, Requirements Document, Architecture Document), where the actual defined business rule (Discount Price is Price - Discount) could be misdefined.

If that's the situation, your unit test means nothing to you.

Additionally, your unit test is another point of failure:

[TestMethod]
public void Test_DiscountPrice
{
    Product p = new Product();
    p.Price = 100;
    p.Discount = 20;
    Assert.IsEqual(p.DiscountPrice,90);
}

Now the test is flawed. Obviously in a simple test, it's no big deal, but say we were testing a complicated business rule. What do we gain here?

Fast forward two years into the application's life, when maintenance developers are maintaining it. Now the business changes its rule, and the test breaks again, some rookie developer then fixes the test incorrectly...we now have another point of failure.

All I see is more possible points of failure, with no real beneficial return, if the discount price is wrong, the test team will still find the issue, how did unit testing save any work?

What am I missing here? Please teach me to love TDD, as I'm having a hard time accepting it as useful so far. I want too, because I want to stay progressive, but it just doesn't make sense to me.

EDIT: A couple people keep mentioned that testing helps enforce the spec. It has been my experience that the spec has been wrong as well, more often than not, but maybe I'm doomed to work in an organization where the specs are written by people who shouldn't be writing specs.