Non-auto supports can easily take duskblade late game

Soraka, Janna, Nami, etc are all good champs for duskblade.

1. Cooldown resets if the enemy dies before the mark finishes them off, so no KS unless the enemy starts running and kiting across the map.
2. It costs a lot of gold and is something you don't want to take before the super high damage items or else you suffer a delay in power spikes (duskblade relies on dealing a lot of damage BEFORE executing, rather than giving damage alone), while supports are sort of forced to squeeze their gold into Locket and other items that no one wants to take
3. Ranged auto attack
4. Hard to waste unless you go crazy with harass and squeeze 50 damage to a 1.5k health champ.

Sure, you'll get rage from people who think they can get a kill after not doing damage against a 20 - 4 ~ % health enemy for two seconds (http://postimg.org/image/a5wn1y1wn/)

But it'd be a fun item to get late game when your damage is low regardless (so don't bother with getting AP or offensive stats) and you'll be killed regardless of whether you have one tank item or not.