...is there any point in making a db so "fat" feature wise? ...isn't a text field where I can serialize anyth anyway I like (with the related logic in my app of course) enough for 99% and things like this a feature that only benefits 1%?
If your database is already doing these checks, then it can simplify anything that builds on them, because you already have huge amounts of guarantees.
Other guarantees you would add are unique constraints, exclusion constraints, check constraints, and so on.
So it might only benefit 1%, but what's the problem with that? If you want to stuff JSON into a text field, no one is stopping you. Of course, now you'd have much less reason to do so, and would be loading a gun getting ready to shoot yourself in the foot...
(but what I'm afraid of about adding all this features is that one day "take the basic set of Postgres features" will become more like "pick a subset of C++ features (that YOU THINK are basic)" and we know where this path goes...)