Should an Editor in Chief publish in their own journal?
There are of course no laws against it but as Editor-in-Chief (EiC) myself I would never do (or at least think twice about doing) it, particularly not as first author. My reasoning behind this is that it could reflect badly on both me but particularly the journal and send signals that all is not necessarily well. I have to point out that in my case we are two and we have an agreement that we do not touch manuscripts from our own spheres of influence (basically departments). Nevertheless, what actually happens within a journal and how people perceive it are two different things and it is my opinion that as EiC one must safeguard the journals reputation as objective. So if the community expresses some form of trust in the editor to publishing in their own journal this could be perfectly fine. 

So when an EiC publishes in "their" journal it is not wrong but potentially "dangerous" and can be perceived as poor judgement. They would jeoparize the journals reputation. So even if thetre is no legal problems there are ethical problems. I doubt anyone would have second thoughts about the quality of a journal if the editor does not publish there, after all if the editor has a solid reputation, associating with a bad journal would jeopardize that.

Can I see any exceptions? Well, co-authorship may be less serious but in the end even that should be avoided. I also think that if an editor publishes a single paper in a thematic issue of the journal (where someone else has suggested a theme) and this is the only paper by that editor in the journal for years, I would not think much about it. Obviously frequency of publications is also a factor.

So, in conclusion, I consider it unwise under most circumstances.