OSNews

When this question is raised it is never taken seriously, why?

It is taken seriously, but there are no easy solutions. It was somewhat amusing to see how Zenja Solana's proposal begins with a bold note:
Protection #2 (prevent application from destroying files) actually has a very simple solution which no OS really uses, which is quite puzzling...
... only to fall apart towards the end. See how the restrict writing to only the apps' directory quickly become restrict writing to apps directory... and oh, the config directory... and oh yeah, the file directory as well. And if you think of this last one, this works only if there is a "hardcoded" place for each filetype, plus there is only one application for each file type (just think of it!). In other words, the only way of protecting important data is to backup them regularly -- there is no easy way to protect user directories without seriously limiting the system's flexibility.

On the other hand, one should not downplay the importance of a multi-user system, like the above post does. Just one example: I run apache on machine, under user:group www:www. Now if a remote attacker founds an exploit in the code of my website, it can probably get access to my system. Actually, I got haXored once this way (and it was my fault, didn't update geeklog for months, even though there were known security vulnerabilities). What happened is that the attacker gained access to files that belonged to www:www, and could write to places where www:www could. Which means, that all my data files in my home directory were safe (because the remote code execution vulnerability would have to be combined with a local privilege escalation vulnerability in apache itself, and the two happening at the same time has a ridiculously low chance). Where I using Zeta, the attacker would have complete access to all my files. That's a significant flaw in Zeta's design.

So, to sum up: the question you refer to is being taken seriously, but the solution is not trivial (it is trivial, but that would mean the end of control over you computer). In the meantime, real security issues (non-encrypted central contact list storage??? - how easy is to solve that for god's sake?) that can be solved and were solved in every other OS should be taken care of, instead of downplaying their importance or impact.