It also helps the farming. Push the lane, farm wraiths, look to gank, rinse, repeat. The faster you can clear a wave, the better.
Granted, as opponents get better at farming under turret, pushing the lane super fast isn't as good as poking to deny cs. But, Kat poke isn't scary at all in general. At least not compared to the poke other champs have. At best, hers isn't a skillshot.
Wait, Q first vs a Karthus? hahaha, oh wow. I definitely need to watch this when I get home. Karthus is my ideal match up for W first. Speed boost lets you practically ignore Defile and Wall while you just run by skittles. This logic applies to basically all the good farmers (e.g., Anivia). Force them back to turret as much as possible. Punish their bad auto attack animations. Dodge skillshots all day.
I don't really care. If anything, the fact some level Q first is beneficial if only since it diversifies the match up. Dealing with Q first Kat and W first Kat are entirely different things.