I can't really see a compelling reason to make this change---it
seems like an unnecessary complication to add to what's currently
a simple function.  Someone who really needs the accuracy can
just use log10.  Perhaps a note in the documentation for log
suggesting this would be useful.

I guess this solution seems insufficiently general:  why 'fix'
log(x, 10) but not log(x, 2), for example?

OTOH, a two-argument log that was guaranteed correctly rounded
(or accurate to within 1ulp) for *all* bases would certainly
be of interest!  But that's a fairly major project...