I suspect that the apps you're seeing on torrent sites either didn't use receipt validation, were distributed *some* other way in addition to the app store, or someone found a way round the receipt validation (it's not 100% secure, but neither is relying on serials).
What you *could* do, is offer a "lite" version of your app with very basic functionality via the App Store. Then make a fully featured "pro" version available for download from your website, with serial based activation.
That way you'd still be getting your product in front of potential customers via the App Store, but you'd be able to sell the "pro" version on your own terms with whatever kind of piracy protection you see fit.
Personally though, I think that Apple's method is probably good enough for most purposes, and better than a serial based approach in my opinion.