People paid good money for the 'Prologue' edition of Gran Turismo 3 and 4, since they were pretty big demos that matched most complete racing games for content. While I'd never pay for an FF demo - or any other demo - I can see why a playable FFXIII demo would do well. It'd make up for some of the endless delays, and it'd let people talk about the gameplay properly before it comes out. That seems to be an increasingly big part of game marketing these days, ensuring that public or journalists have a change to experience the gameplay for themselves before hand.