Some of us have Aluminum in our loops , many commercial manufacturers have aluminum tops, I've seen first hand what a poorly anodized Swiftech block can look like after a couple weeks in untreated water, the minor difference in performance is acceptable when I don't have to worry about that corrosion, as well as bio problems.
Use all copper. You'll get better performance, and the system will last longer in the end. Not to mention copper is a biocide and has no corrosion issues with itself. Its one of those nice engineering problems that solves itself, unlike aluminium and anticorrosive addititves . . .

EDIT: I should also add for accuracies sake many radiators aren't even really copper...
That are commonly used? Like what? (serious question, I'm not aware of any)