|Excellent response - thanks Brian!
I was at the Smithsonian National Zoo in DC last summer and came upon a lovely stand of poison ivy growing up some trees. There was a sign underneath explaining why the Zoo allowed poison ivy to grow here. It is highly beneficial to wildlife and in this case there really was no way for the zoo visitor to come in contact with the plant. The sign also wanted to help educate visitors that there is use for most everything in the world.