[1] http://alexrosen.com/blog/2011/05/sponge-learning/
edit: grammar.
The fact that it is written in PHP is nice because it allows you to use it in existing PHP projects in an environment you are already familiar with. I think the fact that it is written in PHP is less important than the fact that it can be used in PHP (e.g. it would be equally useful as a PHP extension written in C).
But more formally, a self-hosting compiler says stuff about the language per se, see also http://en.wikipedia.org/wiki/Bootstrapping_(compilers).
That it's Turing-complete and can write to a file? I think all it really says is that somebody was hard-headed enough to do it.
And there is nothing bad with this. It is just that many authors don't want to articulate on this and are trying to come up with some artificial rationales.