I think serial keys are necessary. They stop casual copying from being prevalent. Many people are not willing or knowledgeable enough to go through the time/effort to download a torrent, mess with keygenerators and/or no-cd cracks, and then possibly still be blocked from online pay. Without serial keys, anyone could just buy say, an RTS like AoE3 and install it on all your friends computers real quick so you can play together online. There has to be a balance, and I feel serial keys are a nice compromise, since it really doesn't require additional effort on my part, and I can even resell my software, because it is truly mine.