User blog comment:StrategicMagic/Carry by Proxy - Why (Some) Assasins Don't Fall Off Late/@comment-216.36.26.248-20121207075153

It's difficult to express until you've really thought about what "falling off" means and I suspect most people have simply observed that LeBlanc is killing them less often than she used to.

Take two situations: Cassiopeia vs LeBlanc. In this example, LeBlanc can burst for 2500 (not an unreasonable number) and Cassiopeia can deal 700 sustained DPS (also not unreasonable). The below examples will assume that the character is unimpeded by the enemy team, since once you start throwing "what ifs" it never stops.

Target A can take 2300 magic damage (approximately no resist mid). LeBlanc bursts the target down in under a second. Cassiopeia kills it in four seconds.

Target B can take 3000 magic damage (bought a Banshee's Veil which has had its passive placed on cooldown). LeBlanc cannot kill the target in a single burst anymore and must wait for cooldowns. This increases her kill time to somewhere in the market of six seconds. Cassiopeia kills it in five. This is a 500% increase for LeBlanc's Time To Kill but only a 25% for Cassiopeia.

On top of this, Cassiopeia brings an AOE slow, a situational AOE stun (need them looking at you), and will deal linear increases in damage as more targets are in the vicinity. In a dogpile of a team fight, it's not unreasonable for Cassiopeia to be doing 2000 damage per second.

LeBlanc brings a single target silence (or two targets if you no longer care to burst) or a single target/two target slow that can be turned into a root if you're feeling risky.

132.xxx below also makes a lot of very good points, though I do disagree with his Veigar assessment.