I was sad to see this missing too. The difference between `bc` and `dc` is that `dc` uses reverse polish notation (RPN), so instead of `2+2`, you type `2 2 +`. The advantage of (reverse) polish notation is that you can write complex stuff without using parentheses. Once you get used to it (its standard on HP calculators), it is painful to go back to the conventional way of doing things.
The OS X Calculator has an RPN mode. But as far as I know, Spotlight doesn't support RPN.
