Really seriously, that's when I start worrying. Usually, when I hear phrases like that, it's from a salesman, and I walk away. In my experience, "don't worry" means start looking around, it won't work like you're being led to believe.

As my-symbian pointed out there are many ways this might become a problem for a typical user.

Tell me that any app that gets through Extras QA has a hard limit on the total size of files it can install or (at some later point) write in /. Tell me that a user will have to jump through multiple warnings and hoops to install a 3rd party repository. Even given all that, some users will blindly click and install anything from anywhere.

I'm sure that a new user will be quite happy to hear that their new device crashed because / is full, even though they checked and saw 500+MB free where the programs as supposed to go. When they post a thread complaining the response will be that "you only had 65MB free on / to start with, you didn't know to check that because we don't mention it, because you don't need to worry about it".

I'm just saying that it's there so we shouldn't ignore it, otherwise there'll be a new thread every week where someone did something stupid and filled up / because they weren't worried about checking it.