The fact of the matter is that demo versions of software rarely show the more advanced capabilities of the full program. Take the Adobe Photoshop or Sony Vegas trials, those are essentially full versions of the software available for consumers to use for around fourteen to thirty days to get a feel for the program. These trials versions, however, are still heavily limited - this is known as "Shareware" - you download it, install it, use it for a set amount of days and then either delete it at the end or buy the full version.
Now lets compare it to something different. If we compare an actual "trial" version of a piece of software, lets say for example one of the many generic video converters out there to the above Shareware process, you would see that the concept is similar. Usually these pieces of software have a trial time on them, but most have incredibly annoying limits; i.e. Only 3 minutes of video will be converted, which when converting an hour and a half long film doesn't really give you an idea of the overall quality or performance of said piece of software.
In terms of games - the majority of games developers now a days have become incredibly lazy and have skipped having any sort of pre-release or after release demo for any major titles. You spend 40 pounds average on a game, would you want to risk spending that and only playing the game for a few hours before it bored you to tears, or would you rather play it safe and try it before hand? And games developers are wondering why there has been a rise in piracy.
There has been a rise in piracy, but there has been a major fall in demos and trials.