plot3d (and plot, I believe) currently use listofvars to determine if the input expression includes any symbolic variables not defined. This is to prevent Maxima from sending non-numeric data to Gnuplot, and is a 'safety' measure. However, it would be nice to be able to turn this off, with a 'trust-me-I-know-what-I-am-doing'
plot option. See the discussion at