Yeah, that's usually the pragmatic thing to do. Facebook does that with their API, for example.
4xx or 5xx gets you the default HTTP handling for that kind of error. Occasionally - especially in small examples - that default handling does what you want and saves you duplicating a lot of work. More often it gets in your way.
I'd compare it to browser default styling - in small examples it sounds useful, but in a decent-sized site you just end up having to do a "CSS reset" to get it out of the way before you do your styling.
4xx or 5xx gets you the default HTTP handling for that kind of error. Occasionally - especially in small examples - that default handling does what you want and saves you duplicating a lot of work. More often it gets in your way.
I'd compare it to browser default styling - in small examples it sounds useful, but in a decent-sized site you just end up having to do a "CSS reset" to get it out of the way before you do your styling.