It's not 1 charge for the rest of your druid's life. It can be used once every 45 seconds, IIRC.
It means: only 1 enemy can be afflicted by Entangling roots. Not 2, not 3, not more. One 1charge=1 enemy. That's what it is all about.
EDIT: if your enemy is afflicted by ER, the charge is consumed, your target is entangled, and you have to wait for those 45 seconds to pass to get another one.
Edited, Oct 2nd 2008 4:17pm by dragonscale