I think it is more a general thing I am wondering about. I noticed that we pass non strings at other places as well. This behavior is not really as deterministic than building a dedicated string upfront. I guess _Member_ is not a big problem, but we pass for example as well _ExecutableElement_, which toString can change and is maybe to descriptive for a simple error message. Also we are passing Class objects which leads to a toString of "class com.acme.Foo" instead of the simple class name. 
For this reason I was wondering whether we should not be more careful and only pass strings into the Log class (under the assumption that these strings are properly created).
Maybe we should investigate this later. I am fine with how it is now. Just wanted to explain my reasoning.