Ok, so it isn't necessarily the most efficient way to spend your time.  It is usually better to buy your tools whenever possible, but it is a great way to learn.  Of course I am not talking about recreating Nant or CruiseControl.  What I am suggesting would be more in the nature of a test harness for integration testing purposes.  These are strategic tools that will pay for themselves as you move from development into production support. 

One area where these harnesses come in handy are integration points between systems.  This is a notoriously difficult place to debug since you have to re-point servers to developer machines in order to walk through the code.  Your test harness is essentially a mock for the integration point.  If something is failing and you are able to pick up the message and inject it into the destination.

Many developers end up creating these little applications.  We were doing it long before TDD came along.  At that point it was close to the only type of testing.  Try it out and you will find some interesting things about your system.