Objects are made up of fields and methods and in Java these things are typed. Sure, tests that don't deal with this will work just fine 99% of the time, but it's the 1% that worries me. In my book that's where bugs are born.
Yes, I can write methods within my tests that confirm the types for the fields for each class with Perl, but why would I go through all of these contortions when the language I've written the application in does this for me automatically? Why add to your uncertainty when the whole purpose of testing it to reduce it?