OK why aren't uphauls just elastic to keep poles up out the way when stowed?  When do you need strength in the uphaul... I only ever seem to need to pull them down using the downhauls?

Because the poles would bounce up and down uncontrollably.

 The other issue is that when you pull the snodger on the ring wouldn't get to the end of the pole as you'd just keep pulling the pole down until the elastic broke.

You can add elastic to your uphauls, just all elastic doesn’t work 

The pole uphaul sets the pole height and the downhaul brings the ring to the end of the pole in the standard twin pole system.  With the current pole length the uphaul attachment point needs to be very high (near or at the jib halyard block) or else the poles dangle below the boom when stowed.  To avoid this a short length of shock cord doubles the string uphaul for the last couple of feet.