I've handled a patch, and extended both lcm and gcd to take an arbitrary number of arguments -- via functools.reduce, as they are both multiplicative (in the first argument).

Also handled the zero case , so lcm(0,0) = 0 = gcd(0,0)

Use-case-wise, I do a reasonable amount of number-theoretic work and lcm and gcd are always popping up.  If gcd is defined, it's nice to find lcm too.  

For those less well versed in number-theory, the implementation of lcm is not so obvious, and many end up going down a tedious prime factorisation route -- if they get that far.

But is it all worth it in the fractions module ?  They should really be in the math module.