To clarify: I'm not sure trademarks apply to coats of arms at all. The Paris Convention explicitly exempts at least national insignia from trademark protection (and, by extension, possibly from copyright).
Most countries have separate laws for protecting insignia from misuse - which we do indeed not concern ourselves with (see {{insignia}}), as long as the use on commons itself is legal, and it's possible to legally use them on some Wikimedia project.
My recollection of that (maybe inaccurately) was that that came down to copyright issues, not trademark? A photograph or redrawing of a logo still infringes its copyright, after all. I don't remember any of the disputed logos being out of copyright.
As far as I remember, we decided not to have any trademarked logos, copyrighted or not. Many logos could be considered PD because they are trivial - in fact, the German WP allows such logos, and is quite liberal with the interpretation... a dangerous course, IMHO.
I believe I am correct that Commons does not generally concern itself that the appearance of a trademarked element in a photographed scene may make a photograph unusable in some (but not all) commercial contexts, however.
Yes - logos appearing in a larger context are OK. Just *how* large that context must be is subject of interpretation... a street scene that contains some logos somewhere is probably ok. A picture showing nothing but a Coca Cola can? Not sure about that.
-- Daniel