I'm not sure if Scizor should be as high as it is, but it's worth pointing out that part of the reason for this is Scizor's flexibility in terms of what kinds of teams you can put it on. Or I should say, you can pretty much put Scizor on any team and it will do something. I can't really think of a type of team, actually, that Scizor couldn't really go on - it's almost always an asset to some degree. Compare this to Politoed - For obvious reasons, Politoed can't go on a sand team. Politoed can't go on a sun team. If you're running toed, you have to use a rain team, and that restriction affects it's usage. Comparatively, Scizor's ability to appear in any weather or none at all affects it's usage, but the other way around.
Does that make sense?
So it's not really Scizor's power that drives it's usage per se, just it's ability to be generally useful in a wide array of situations. Like I said, I'm still not sure if Scizor deserves the #1 spot (and I'm certainly not defending it), but I *think* this is the reason behind it's placement.