Doesn't lag you much at all and gives you that extra damage needed to finish a kill when they're trying to run away.

You can also use it well when you know which way you fled, you can quickly run back in and knife again. Knife, flee, knife, flee can be extremely effective in the right situation.

I've played a couple thieves and would probably always take knife but it's not something that you couldn't live without if you really needed the edge points for a different build plan.