A study done about a couple of years ago actually discovered that games would sell more on release day if they DON'T have a demo beforehand.
That and I'm sure that there would be some broken stuff on the demo like Johnny and Lex Luther block infinites from Mk9 and Injustice.
I'll never forget a video I saw when Mk9's demo first came out. It was of three well known Japanese fighting game players doing the block infinite on the demo and just laughing at NRS and MK9. It honestly hurt a little bit like someone was laughing at my little brother or something.
I'm glad there's no demo, really.