I was doing some type conversion routines last night for a system I am working on. One of the conversions involves turning string values into their DateTime equivalents.
I think it's there for completeness and that you get an explicit InvalidCastException when calling Convert.ToDateTime with an object that is a bool.
If you look at all the members of Convert, you can see that overloads have been included to accept all the basic types for conversion to each of the other basic types, with InvalidCastExceptions being thrown when there is no sensible conversion.
I guess they thought this would be more meaningful than just not having the overloads there at all.