That's fine for black-white transparency, but less good if you have intermediate values in both transparency maps. For instance, one map giving 90% transparency and another giving 50% transparency. Minimum will give you the most transparency at a particular point, so where the two transparent areas overlap you get the 90%, rather than the 95% a multiply would produce. This is probably more useful for variations in the specular intensity or reflection size. And remember that a hole doesn't have a specular reflection, while a piece of glass does.