I agree with others here that have stated they get the most insight from real-time play throughs. Watching someone "noodle" through raw patches is awfully telling, and is usually not too dependent on their ability to play the piano. While the thoughts of the walkthrough creator might be valid, the real value in these videos comes from being able to hone in on the raw sounds, how the transitions are responding to the midi data, the quality and depth of the recordings etc., and then making your own conclusions.
While poorly executed demos obviously affect the perception of how good a library is, I think that the opposite can be equally frustrating and just as common. Midi wizards are consistently capable of squeezing absolute gold out of absolute turd... (that's an interesting topic for another time). This also provides an inaccurate perception of library quality. For example, although I am a huge fan of Hollywood Strings / Brass (still some of my favourite commercial libraries), I am not sure I have ever heard any other examples that come close to matching the quality of Thomas Bergersen's demos for those products.
Hearing demo tracks is useful in some ways, but IME it is a poor judge of how effective a library is. I would argue that demos showcase the composer's midi (and composition) ability more than the library's quality itself. I have been turned off libraries that are actually brilliant because of bad demos. I have also bought libraries that I have instantly regretted after playing the first note, because of amazing midi wizardry. I'm not interested in hearing how someone uses the tool, I want to hear the
actual tool for what it is...
I am constantly having to check myself when listening to demos:
do I actually like this library? or is this just a badass melody? or is the orchestration just perfect? or is the mix god tier? Demo tracks bring in so many other variables that in the end distract from our ability to make a solid personal judgement.
All that aside: it's a clever marketing tool and they will obviously be here to stay. You'll find me stalking YouTube for walkthroughs though
(Unless there's a playable demo of course).