Last I checked, the average champion in league has access to at least 1 form of escape on a high cooldown, and slightly less have an additional one on a much lower cooldown. You land human E, you use human Q and W, they flash, you use spider E to get back on them, they use another escape, you use Q to get close again. They've now expended all resources, you've maintained all of yours, and you're still in close enough range to use W.
All of my assumptions of early points in W is assuming that you will get at least 1, usually 2, maybe 3 ganks off before level 6. Assuming all of this, blowing all of your spells in an attempt to burst somebody down can leave you walking away, in a bad case scenario, from the lane with either the laner's resources blown, but no kill to show for it, or you got a kill, but probably blew flash. Plus you're hinging all of this on a narrow skillshot that could have been dodged anyways. A measured use of spells will keep you well within range of your target for at least partially optimal use of W, still get the early damage increase from W, get a faster and safer clear for less volatile ganks, and expend less of your own resources to get a kill off a gank.
I'm not arguing that W is the first max, I'm arguing that for the early levels pre-6, it's better to have points in W because of a greater return on investment. It's always a Q max first because of Q being used more than once per gank on average past level 6, but you're never going to take advantage of it's cd at early levels. You'll get one rotation off at most, so you want the greatest return on damage in 1 rotation.