I want to build the extra shit and I'd like to my friends to invade me and vice versa. That's not a lot to ask for is it? Why is it one way and no other way?
If you get rewards/experience for successfully defending your base, I could see that being exploited if you and friends just go back and forth attacking without really attacking just to get the defend points, or whatever. Not saying YOU would do that but, you know, the figurative "you".