I think the reason pros dont use (vs other pros) is:
1 Silence is so PWNAGE
2 Black arrows nice to
3 while life draining you can run or anything makeing you an easy target for the other players army, this would probably be differnt if dark ranger was melee or had a defensive ability.
I use Dark Arrow and Life Drain when i go with her though normally
i luv her charm and how fast it recharges, Great for stealing things like tauren, chimeras, frost dragons, or any other high value units.