There is still a problem: the analogy, that log/logf is similar
to write/writef, is not right anymore. Please correct me if I'm
wrong... but AFAIK write converts every argument to string and
the output device has no mean to choose a preferred
representation of the arguments.

It will be a surprise if I replace
log(a, b, c);
with
logf("%s%s%s", a, b, c);
and possibly get different results.

If you are just using default logging, they are the same. Point is that it allows for different results based on logging configuration/implementation. The logf version says the logging output has to be exactly that string, in all instances. The other says here is some things I want logged, and the logging can decide how to output them.