TestCase attribute requires all test parameters to be specified, even the ones with default values.
Try this example test

        [TestCase("va1")] // this does not work
        [TestCase("val1", false)] // this works
        public void Test1(
            string a,
            bool b = true)
        {
            Assert.Pass();
        }

The test above will say "Wrong number of arguments provided".
BTW. I'm using NUnit 2.6.3

I know this is not the best idea from the purity standpoint but still this would be occasionally useful.
I leave this to your decision as you have more experience and skills.