A good clean hickory stave of the dimensions you specified, should make practically anything but an elb.
Is this from personal experience that youve tried? Or are you just regurgitating dogma you heard told/read by some ole timers? Hickory can most certainly make a nice elb...its been done.
With that said id make an ELB!!!! 
I haven't made an elb, but I have made a few cherokee style longbows from hickory. It is a wonderful wood, and I would definately try an elb from it myself, but I'd hate to be the guy that said "yeah go for it" if his bow take some major set or chrysals or something. I'm not saying I think that's likely, but it is at least possible. Unless I really wanted an elb and nothing else would do, I don't think I would risk it on such a wide stave with so many possibilities. To me, it just seems wasteful somehow. You are right to question dogma, and you made me stop and think about it, but I stand by my statement.
