True, very true. I hate when I mistakenly hit enter and it opens a blank calculator. One would assume that the calculator has an api where you can send queries to it, maybe it doesnt have one for 'open with predefined equation'
I wouldn't assume that — a Cocoa application process is an insane amount of overhead for a simple arithmetic expression evaluator, so who'd bother using it? Assuming Spotlight uses the same underlying evaluator, it's a safe bet that it's implemented as a library shared between the Spotlight search UI and the app. To be even more specific, on 10.8 at least, both Calculator.app and Search.bundle link to a private framework called Calculate, which may be relevant.