Zinc plating can help reduce significant corrosion of alu even when the plating is scratched or penetrated - the zinc and alu are still in electrical contact and the zinc will still oxidize preferentially. I'm reasonably convinced that Swiftech's top poses no real risk of a corrosion disaster in the working life of a real block. Nevertheless, a copper top seems a more obvious choice.

It's possible that the alu+cu risks are somewhat overstated: if your water is de-ionized, and your alu has some sort of protective coating, the corrosion problem should be ... tractable. The life of a block is rarely more than a year or two. You have to look very carefully at the horror stories, question all the details and see how the problem arose.

Maybe if somebody were heating up tap-water in their loop they'd quickly see serious problems. Maybe they bought deionized water that wasn't, maybe there was a residual substance (such as flux) that contaminated their water...

What we do know is that there is a very small number of verified cases of mixed metal disaster in water-cooling loops but those few cases are 'sensational' enough to make an impact on everyone - despite the fact that more people probably lost kit to cracked Tt perspex blocks, o-rings damaged in mis-assembly, dodgy quick-fit connectors, and dead Mag2 pumps.

I'm not saying don't worry about mixed metals, just that in the case of this block the reaction is a little extreme. Swiftech did the best thing they could to protect the Alu, it's just not obvious why they chose to use it in the first place.