Orb walking also puts you in the same ranges (ie past the creep wave, if you're making use of the fact it doesn't draw aggro) and curse/word have much longer effective ranges. In addition, pulling aggro from creeps is not always a bad thing; in fact, in many dual lane situations, pulling the lane backwards a tad is beneficial.
For one level of curse, you deal around 15 damage and 8 mana loss at the very least plus whatever mana they burned to get the curse off, or you do around 100 damage and 48 mana burn, or some interim, for 75 mana. Conversely, one level in Glaives adds around 10-12 damage per 15 mana, which results in around 50-60 pure damage for the same mana cost as a level one curse of the silent.
The differences in damage per mana cost increase severely as you level curse in comparison to glaive, as each level only adds 20 mana to the cost for +15 damage and +8 mana drain per second per level. Curse also features 550 (800 cast range, 350 radius) more range than your autoattacks, and is arguably safer than orb walking. If you're in a mid situation, forcing the opponent to cast a spell to get rid of curse or otherwise take damage is highly advantageous. I won't argue the smoke ganks, but those don't occur in pubs until far after the leveling issue is moot and silencer isn't picked in pros. I'm not arguing that orb walking is cool beans, but math wise you're generally going to get more out of levels in Curse now that you don't need glaives for int steal.
