rednation, to summarize it really depends--you can sometimes find low (virtually no) WE kids who you would take even if they did not improve, either as role players or they just are so much better than you have. But as a rule I'd agree that WE wants to be over 20, and preferrably over 40 if you want/need/expect improvement, unless they can start right now.
Kids under 20 WE I always ask myself would I still want them if they did not improve at all. If you can answer yes, then any growth they have is icing on the cake. So don't completely neglect them either. But "projecting" their potential out when they have a low WE is not realistic. I have a kid who is a junior and up to a 19 WE and high potential in most categories, but he isn't good enough to command the minutes and so he will never even approach his potential. Another kid on another team had a 13 WE when he came in, left with a 19, but he played a key role on the team and actually was averaging double digits off the bench (after starting ~ 1/2 of his junior season until he was overtaken) his senior year. So don't disregard them completely, just understand the risk and know where they fit into your team.
However, the vast majority of kids will need to improve to become impact players, so just evaluate with your eyes wide open and you can make any WE kid fit.