[RHSeeger]

> Nobody in my experience is testing add("a", FooObject) for a function only meant to take ints or floats, to make sure it only takes ints or floats.

If it's a dynamically typed language and you want to be sure that your method throws an error on invalid types (rather than, say, treating the string "yes" as a boolean with a value of true), then unit tests are a good use for this.

I would argue that failing fast (for cases like that, where an input "could" be treated as the type you want, but almost certainly the caller is doing something wrong) is a positive thing.