I do think quality video clips can tell you a lot about some gear, depending on how well you jive with the creators style as well. It is hugely important.
Jon's videos are quite useful to me, because he goes for similar tones to what I go for. Conversely, someone like Burgs tells me absolutely dick all about gear, because he just doesn't play anything like me, doesn't go for the tones I go for, and isn't any kind of frame of reference for guitar playing in any shape whatsoever - for me, personally. Loads of people really rate his videos though.
That doesn't just go for amps, but it goes for effects too.
Not to go too off topic, but you can't seriously tell me that my Volante video here isn't useful and doesn't give you any idea of how the thing works and sounds:
I definitely agree you don't get the full picture, but they're not useless and they have value outside of mere entertainment.