I think it needs to be cheaper like Madred's Razor, which is a much better farming tool. Combined with lifesteal, and heroes like Trynd and Yi will never need to leave a lane, as you lifesteal off of the +500 dmg procs.
Just my 2 cents, a 1000g item, which you can get pretty early on, is half the cost of tiamat, and pays itself off just as quickly. I've soloed Dragon using Madred's + Lifesteal.
I was wondering if anyone had tried Tiamat on Kayle? I'm getting into her and I thought that maybe the splash would stack with her ranged stance, making farming easier for her, since it's pretty tough till she gets either Sunfire cape or Atama's.