Agree to disagree. Of the top of my head I can think of several useful things where a public (private) address would be problematic:
1. you want to do something like wiki leaks
2. you want to publish your thoughts anonymously, let's say you are from the LGTB spectrum and want to engage with people by building a forum or blog to communicate with them -> this could lead to potential problems with family, friends and work
3. you have political views and publish them. Now say someone disagrees with your views and is potentially aggressive. Do you want them to know where your family lives?
4. you do not wan't annoying calls by people who crawl the DENIC records (happens to me regularly)
I agree with you that a person should be liable for the stuff they do, but should also be able to engage in open discussion while protecting their privacy. FWIW If someone does bad stuff on a .de domain there are several options:
a) take down the domain via DENIC, or b) take down the domain via ISP or c) take down the domain via Hosting provider
If you really did illegal stuff, I am with you and someone (DENIC, ISP or Host) probably should have the knowledge of the domain owner which could be subpoenaed to lawfully prosecute someone.
making a single person personally liable for content that's hosted on a domain is problematic, to say the least. Go down that path and you eventually end up where the chinese government is now. Where you need a "license" from some ministry to operate an http daemon and you need to hire censors to police all user generated content that disagrees with an authoritarian regime's politics.
This is a major criticism I have with many other domains, I don’t know where to send a C&D, or whom to sue if they violate my rights.
And requiring only commercial entities to have these things also makes sense.