Are defensive land units useless?

It seems like defensive land units are fairly useless. Do missile units respond even if not attacked directly? What do other units do besides sit in the way, they don't seem to react if say you're getting painted. So what use are they? Is it that you need to cover every inch of your country with them to have a defense? Seems to me like too many supplies to be worth it.

From what I can tell, there are a few bugs that allow Naval units to attack but that because most don't have Navy defense.

I havent had to test it but I think the more defense units you have, the harder it is to have everyone killed as you can keep your higher war index which prevents easy takeovers.