First of all, look at Apache log4net:
http://logging.apache.org/log4net/[
^].
I think decoration components with logging is too heavy weight and too much if stiff coupling. I would allow it for an optional feature where logging is often of always useful. In general case, this is no good.
The use of singleton if way more adequate. I would agree that this would be a classic case of singleton usage. "Hard for unit testing?" Hm. I suspect this is just a result of a sick fantasy of some person who does not actually do unit testing. I don't see any specific problems for unit testing here. I can imagine that I'm missing something though. In this case, let this person write to me and explain where is that hardness; I would greatly appreciate it.
—SA