Well, smacking someone on the head is pretty unpredictable in whether or not you're really knock them out, kill them, or just tick them off, plus it would be pretty hard to guess how long they'd be out for. Using ether, chloroform, or whatever was in the syringe in Contracts however, you could much more efficiently gauge roughly how long someone would be out. I liked how in the older games, the longer you soaked the handkerchief and held it over the persons face, the longer they'd be out. It's actually a bit disappointing to hear you can just beat them into unconciousness if I'm understanding you correctly.
Jeez, taken out of context, that might have been one of the most disturbing paragraphs I've ever written.
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If it's randomized between killing your victim, ticking them off, and successfully knocking them out I'll be one happy gamer.