Hi,
the nature of Alu is that it forms a thin layer of Alu Oxide which protects the Alu underneath it. We are talking microscopic amounts, but technically you are removing a protective coating when you polish. You'd never polish enough to harm the cranks; but as the other posters pointed out; it's not a priority for most. My 'kink' is I like to clean my chain every time I use it. I love the buttery smooth feeling of a perfectly clean chain. Basic maintanace things, checking air pressure, cleaning the chain, etc will pay you back for your effort. I leave a layer of grundge on my bike hoping to make it less attractive to thieves.