ALL titles should have demo. PC developers have gotten complacent and lazy and rely on their brand names to sell their games without having to actually create a demo to give gamers a feel for whether it's worth their hard earned cash.
PC gamers similarly have gotten lazy by just buying games without a demo. Consider the success of games like Quake, BF, etc - these games had massive, thriving communities before the game was even on the shelves, all because they had a freely available demo, which served as the best thing to market their game.
It's a sad state of affairs and it won't get better until PC gamers simply stop buying games until a demo has been released. Otherwise they simply won't change the process.