Malady: early-mid game, single target, 2v2:
At this point of the game, a fight can last long enough to make malady count. If malady is your 1st item, you're increasing your DPS by 50% (minus other speed boosts per build & character). In the 2v2, you're not only increasing YOUR DPS, but your allies as well. On this note, I'm pretty sure you get all the credit for malady damage caused by anyone, as I've gotten killing blows on enemies I certainly did not last hit (but had malady debuffs on them).
Mid-Late Team fight:
Far less useful. Unless you're twitch and hitting the whole team at the same time with your basic attack, the damage proc is trivial. Even as Twitch, it's not a great late game item... there's just too many better attack speed items.