Aluminum cases can help spread the heat around if it is concentrated in one area (RAId array, for instance). They also look EXTREMELY cool, which I think is the major selling point.
Why you should buy an aluminum case:
It looks cool
It's light
It "helps cool": the system
You're pretty much paying for the look, but if you like that, go for it. Not really enough on its own, though.
"If you teach a child to read, then he or her will be able to pass a literacy test" - George W.