This topic has been discussed at great lengths in Vinylengine.com. According to a Warner Bros mastering engineer who is a member out there, half-speed mastering is merely a marketing gimmick. It sounds nice only because a lot of care has been taken to master the record. Half speed mastering was required to overcome the limitations of record cutting styluses at that time. Post the 70's however, with helium cooling technology (I think), cutting stylus can cut at very high frequencies beyond 20khz.
