Coverage for this R6RS feature is currently sparse: only Gauche, Chez, Vicare, Larceny, Ypsilon, Mosh, IronScheme, KSi, RScheme, Rep support it. But it is convenient when working in bases other than e such as 10, 2, or 16, and it is just a few extra lines of code, since (log z b) => (/ (log z) (log b)) for arbitrary complex numbers z, b.
WG1 decided to adopt this extension.