Sometimes, you get more out of a combo if you level skills equally. For example, prioritizing E over Q on Wu decreases his initial burst while allowing him to keep whacking at an enemy. But getting Q first means his initial burst is higher, but he doesn't have the AS to keep hitting the enemy afterwards. By levelling them equally, you get a bit of burst and a bit of DPS afterwards, which adds up to a bit more than either of the former options.
It's fine on certain champions. Especially Junglers who may want a few points in their best clearing tool, but then start putting points in other abilities (I do this with Sej).
Most of time though an ability is maxed quickly because of mana. Because if you invest all your power in one ability then your mana to power efficiency goes up. Leveling things evenly means you have to burn all abilities to have a similar payoff. This is important for poking, trading or waveclearing.