Because it deals damage based on current health it won't increase damage much against squishy characters, but it would deal signification damage against champs like Mundo and Volibear, it would be an ideal item against tanks who stack HP when you have a single target nuker (LeBlanc). As it does damage based on current health the dot can't finish the target alone.
If multiple spells are cast at the same time the strongest (single target) dot will be prioritized during its duration. e.x. Kassadin casts Q then after one seconds he casted E. 10% will be applied for 4 seconds, then the 5% for one second. (2.5% each second for 4 seconds and 1.25% for 1 second).