If alum rad's were so ****y as you put it, why do all top racers run them? (nascar, trans-am, etc etc) or those with highly modded high HP street motors?.
The biggest reason is weight savings. And also, because most high end race engines use aluminium heads and mixing copper/brass radiator with aluminum heads is not recommended because it promotes galvanic corrosion. If you have an aluminium water block then use an aluminium core radiator but if you have a copper water block then use a copper core (this will stop electrolysis/galvanic corrosion from happening) though in the end copper gives the best heat transference. It's a fact that copper transfers/dissipates heat faster than aluminum, but copper cannot hold as much heat as aluminum.