"No shit!" you'll probably say. Who doesn't think it's important?
But "scene selection" probably means a hundred different things to a hundred different editors, so who knows if any of us are really on the same page in this thread? I can't even bring myself to finish another AMV because I'm so picky about scene selection. Does this clip I'm using logically follow the one that preceded it? Does it make sense from a narrative, compositional, and sync-related point of view? If not, then I have a very hard time leaving it on the timeline.
I get the sense that lots of editors just pick a handful of "cool"-looking clips, throw them on the timeline, and stir them around for a few hours until there's a semblance of a finished video. Sometimes this actually works! But at best, it's a dizzying array of high-quality opening credits footage that didn't require a lot of thought to put together, but still leaves viewers commenting OMG! This is made of win! LUV IT.

I've tried to be very selective about scenes to use, but I'm not sure this is apparent to every viewer. Maybe plenty of videos that I've been quick to criticize for their lack of attention to scene selection were actually labored over very intensely, their editors desperately trying to pick not just "the best scenes," but scenes most suitable for their video's specific concept and theme. So who knows. No one can say this isn't a really, really important part of making AMVs. But even when it's done well, or at least given great attention to by an editor, it's not always going to be explicitly apparent in the finished product.
blah, blah, blah, cool story bro, tl;dr, etc.